Understanding the characteristics of those who use Syringe/Needle Exchange Programs (SEPs) and pharmacies is important for program planning. This study examined the factors associated with where injectors get sterile syringes and if distance to a SEP site is associated with syringe acquisition. Three hundred and four adult heroin and cocaine Injection Drug Users (IDUs) participated in an epidemiologic study of risk factors for HIV/AIDS. Multinomial logistic regression examined the association between where IDUs obtained most new syringes (a SEP, pharmacy, or other less safe source) and other key variables. Results indicated that the relative risk (RR) of getting most syringes from a pharmacy (versus another source) was higher for: males, whites (versus African-Americans [AA]), and those with at least a high school diploma and lower for: those reporting receptive sharing of injection equipment. Results also indicated that the RR of getting most syringes from a SEP (versus another source) was higher for: those injecting more frequently (1-2 times/day vs. <1 time/day and 3 times/ day vs. <1 time/day), those who have been arrested for drug paraphernalia in the past six months, and those using syringes for 1-2 hits (vs. 3 or more hits) and lower for: those living farther away from a SEP and those reporting receptive sharing of injection equipment. Results also indicated that the RR of getting most syringes from a SEP (versus a pharmacy) was higher for: those injecting more frequently (1-2 times/ day vs. <1 time/day and 3 times/day vs. <1 time/day) and lower for: whites (versus AA), those with at least a high school diploma, and those living farther away from a SEP. SEPs and pharmacies may serve different populations of IDUs and may play complementary roles in preventing infectious disease transmission. Distance to a SEP may be important for where IDUs acquire sterile syringes.

• Syringe exchange program
• Pharmacy
• Harm reduction
• HIV/AIDS
• Injection drug use
• Injection equipment sharing

Mancha BE, Whitaker DE, Severtson SG, Nandi A, Latimer WW (2014) Factors Associated with Acquiring Syringes from Syringe Exchange Programs and Pharmacies among Heroin and Cocaine Injectors in Baltimore, Md. J Subst Abuse Alcohol 2(3): 1017.

SEP: Syringe/Needle Exchange Program; IDU: Injection Drug User; SD: Standard Deviation; RRR: Relative Risk Ratio; 95% CI: 95% Confidence Interval

In the United States (US), chronic illness and infectious disease as well as increased morbidity and mortality have all been linked to injection drug use. According to recent studies, approximately 12% of the new cases of HIV in the US, about 6,600 cases, are among Injection Drug Users (IDUs) [1,2]. Further, injection drug use is also considered responsible for a significant proportion of hepatitis viral infections [3-6].

Injection drug use is an accepted etiological agent of infectious disease. Prevention efforts should then focus on approaches that reduce the risk of transmission. Research has revealed that the practice of sharing injection equipment accounts for a considerable proportion of blood borne viral transmission [7]. Public health authorities and researchers define sharing injection equipment as using syringes or drug solution (frontloading or backloading), rinse water for cleaning syringes, cookers or water for drug mixing, and cotton filters after someone else has already used them [8-12]. The US Department of Health & Human Services and the National Commission on AIDS established that effective risk reduction, for those who are unable to stop injecting drugs should include single use of sterile injection equipment and appropriate discard of used materials [8,12]. In order for IDUs to follow this recommendation, they require access to sterile materials.

To combat the growing spread of HIV/AIDS among IDUs some states began to implement harm reduction measures aimed at getting injectors access to sterile injection equipment and getting used equipment out of circulation. Legal sterile syringe distribution policies, such as Syringe/Needle Exchange Programs (SEPs) and over-the-counter sale of syringes at pharmacies were implemented. There is extensive published evidence for the effectiveness of SEPs and pharmacies in reducing the spread of HIV infection among IDUs. For example, SEPs and pharmacies provide greater access to sterile injection equipment [13-17], lower the risk of injection equipment sharing [14-16,18-22], and play complimentary roles in decreasing adverse injection-related outcomes [15,16,18,20,23-29]. However, despite the empirically supported benefits of harm reduction programming [10,11], a national policy has yet to be implemented. Furthermore, there are state regulations that give pharmacists discretion over syringe sales and laws that impart penalties for possession of syringes without a prescription. In addition, only recently have prohibitive policies regarding federal funding for SEPs been lifted. Despite some political and social opposition to harm reduction programming, most public health officials and scientists favor this public health approach to infectious disease prevention.

Because SEPs and pharmacy policies are effective at reducing the transmission of infectious diseases, researchers are interested in SEPs and pharmacies and in the characteristics of their patrons. Understanding these characteristics may advise policy-makers on policy decisions; they may also help researchers, program administrators, and program developers design more effective programs to reach those in need of services and target existing programs in need of update.

Furthermore, the research base on the determinants of SEP and pharmacy use among IDUs is growing. Studies have shown that SEPs and pharmacies attract different groups of drug injectors. For example, SEPs attract users who inject more frequently [30-35] and have an unstable lifestyle [31,33,36]. Pharmacies attract IDUs who are less disadvantaged [37,38]. While a number of studies have compared SEP users and non-SEP users (for a review see Wodak and Cooney, 2006) [21], few studies have examined geographic distance to sterile syringes and injection risk behaviors. Preliminary findings from that research indicate proximity to a SEP site is an important factor to consider when examining injection risk behaviors. For example, one study found that IDUs living closer to a SEP were more likely to use it compared to those living further away [39]. Another study found that the odds of using injection equipment after someone else increased with each mile increase in average distance from a SEP site [40]. Two other studies found that spatial access to syringes and arrest rates were associated with injection equipment sharing [41,42].

Existing studies comparing SEP users and non-SEP users tend to be confounded by the fact that some non-SEP users may get their syringes from verifiably safe sources, such as pharmacies [21]. Further, by grouping pharmacy users (pharmacies are a safe source of syringe acquisition) with non-SEP users, the protective effect of SEPs may be underestimated. Similarly, studies examining pharmacy use or laws governing pharmacy access to syringes generally do not consider SEP use, which in turn may underestimate the protective effect of pharmacy access to syringes.

Accurate knowledge of the characteristics of IDUs who use pharmacies and SEPs, as well as the impact of geographic distance on injection risk behaviors and SEP or pharmacy use, is needed for a comprehensive infectious disease prevention policy. This study focuses on the factors associated with where injection drug users in Baltimore, MD get most of their new syringes, from a pharmacy, SEP or other source, with a special focus on whether geographic distance to a SEP site affects IDUs’ syringe source.

Participants and procedure

Injection and non-injection drug users were enrolled in the Baltimore site of the NEURO-HIV Epidemiologic Study [43] an epidemiological examination of neuropsychological, social, and behavioral risk factors of HIV, Hepatitis A Virus, Hepatitis B Virus, and Hepatitis C Virus. To be eligible for the study, participants had to be aged 15-50 years and report using injection or noninjection drugs during the previous 6 months. Participants were enrolled between 2002 and 2004 using multiple recruitment strategies, including street outreach, outreach at local syringe/ needle exchange programs, advertisements in local newspapers, referrals from enrolled participants (snowball recruitment), and referrals from social service agencies. The Johns Hopkins Bloomberg School of Public Health Institutional Review Board approved the study protocol in 2001 and annual reviews and human subjects’ approvals have been maintained. Informed consent was obtained from all participants.

The present study included 304 current regular injectors of heroin, cocaine, or heroin and cocaine (speedball). Only participants reporting a lifetime history of regular injection of heroin, cocaine, or speedball (ever injected daily or nearly daily for three months or more) and injecting in the past six months were included. The present study was also limited to AfricanAmerican and white participants. Additionally, only the cross-sectional baseline data were used in the analyses.

Measures

The baseline assessment consisted of a standardized HIVRisk Behavior Interview that was originally adapted from a similar interview used for the REACH study [44] and included interviewer-administered questions about demographic characteristics, drug use, injection practices, and HIV-risk behaviors. The dependent variable was where participants reported getting most of their new syringes, from a pharmacy, from a Syringe/Needle Exchange Program (SEP), or from another source (including wife, husband, boyfriend, girlfriend, family member, friend or acquaintance, diabetic or their relative, drug dealer, syringe dealer, or some other place).

Several independent variables were shown in the literature to be associated with where injectors acquired their syringes. Demographic characteristics included age, African-American or white race, gender, and education (less than high school or GED vs. high school diploma or higher). Injection risk behaviors included any receptive sharing of injection equipment (using syringes, drug solutions, including backloading and frontloading, or works, including cookers, cottons, filters, and rinse water after someone else has already used them) in the past six months, how many times a syringe was used for drug injection in the past six months (one or two times vs. three or more times), and frequency of drug injection in the past six months (less than daily, 1-2 times per day, or 3 or more times per day). Other variables included having been arrested for possession of drug paraphernalia in the past six months and distance from the participant’s residence to the nearest syringe/needle exchange program (categorized into tertiles).

The variable measuring distance from the participant’s residence to the nearest SEP was created using the reported residential address of each participant and the address for the nearest syringe/needle exchange program site within Baltimore City and in operation during the study period. Baltimore City Needle Exchange Program data was obtained from the Baltimore Substance Abuse Systems, Inc [45]. ArcGIS was used to map participant addresses and SEP locations. The Hawth’s Tools extension for ArcGIS was used to obtain the distance between each participant’s residence and the nearest SEP site [46]. The resulting spatial distance was then categorized into tertiles for analysis.

Statistical analyses

All analyses were performed using Stata, version 10.1. Initial unadjusted analyses (cross tabulations and multinomial logistic regressions) examined the relationship between independent variables and where injectors obtained most of their new syringes. Additional exploration included adjusted multivariable analyses. Examination of the unadjusted results revealed a very similar pattern of results compared to the adjusted results, therefore only the adjusted results are presented here.

Multinomial logistic regression was used to examine factors related to where injectors obtained most of their new syringes (pharmacies, syringe/needle exchange programs, or other sources). Analytical results revealed a cumulative missing data percentage of 20.4%. Using methods outlined by Van Buuren (1999) [47] data missingness was examined and it was determined that the missingness was not missing completely at random but missing at random. Therefore, missing values were imputed using multiple imputation, which accounts for missing data that is not missing completely at random. The multiple imputation methods used chained equations via the ice command in Stata, version 10.1; these methods are described in more detail elsewhere [47,48]. Data were imputed and analyzed multiple times (m=5). Estimates from the multiple analyses were then combined using Rubin’s rule of inference [49]. Multinomial logistic regression results are presented as relative risk ratios.

The sample was 63.5% male, 70.7% African-American, ranged in age from 17 to 50 years, and had a mean (SD) age of 31.5 (7.3) years (see Table 1).

Table 1: Characteristics of injection drug users in Baltimore, MD and cross tabulations by where they obtained most of their sterile syringesa .

a: Column percentages are presented; Missing data are not presented for cross tabulations; Seventeen participants (5.6%) had missing responses on where they got most of their new syringes; SEP: Syringe/Needle Exchange Program

Table 2: Multinomial logistic regression results examining the factors associated with where injection drug users in Baltimore, MD obtained sterile syringes.

a: RRR, relative risk ratio; 95% CI, 95% confidence interval; b: Other place is from a partner, friend, family, diabetic, drug/syringe dealer, or other place; c: SEP, syringe/needle exchange program; *P<0.05.

Participants were mainly heroin users (56.5% had ever used heroin regularly and had used in the past six months), 39.1% were speedball (both heroin and cocaine) users, and 4.4% were cocaine users. Participants had been injecting drugs regularly for a mean (SD) of 8.9 (7.4) years and 22 (7.9%) were seropositive for HIV. Over a third (33.9%) of the sample reported obtaining their new syringes from pharmacies, one quarter (26.0%) obtained syringes from SEPs, and 40.1% obtained syringes from other sources, including wife/ husband/boyfriend/girlfriend (N=6), family member (N=3), friend or acquaintance (N=8), diabetic or their relative (N=14), drug dealer (N=11), syringe dealer (N=60), or some other place (N=3).

After adjusting for other covariates, the multinomial logistic regression model revealed that the relative risk of getting most syringes from a pharmacy versus from another source was higher for males (versus females, Relative Risk Ratio [RRR]=3.08, 95% Confidence Interval [CI]=1.61-5.91), whites (versus African-Americans [AA], RRR=6.73, 95% CI=2.58-17.56), and those with a high school education or higher (versus those having less than a high school diploma or a GED, RRR=2.53, 95% CI=1.29-4.98). Also, the relative risk of getting most syringes from a pharmacy versus from another source was lower for those reporting receptive sharing of injection equipment in the past six months (RRR=0.44, 95% CI=0.22-0.88).

Moreover, after adjusting for other covariates, the relative risk of getting most syringes from a SEP versus from another source was higher for those injecting one or two times per day in the past six months (versus those injecting less than once per day, RRR=4.23, 95% CI=1.68-10.68), those injecting three or more times per day in the past six months (versus those injecting less than once per day, RRR=4.68, 95% CI=2.01-10.91), those reporting that they had been arrested for possession of drug paraphernalia in the past six months (RRR=4.74, 95% CI=1.26- 17.80), and those reporting that they used a new syringe for one or two hits (versus those using a syringe for three or more hits, RRR=3.34, 95% CI=1.27-8.76). In addition, after adjusting for other covariates, the relative risk of getting most syringes from a SEP versus from another source was lower for those living the furthest away from a SEP (those in the third tertile, compared to those living nearest to a SEP, in the first tertile, RRR=0.31, 95% CI=0.12-0.79) and those reporting receptive sharing of injection equipment in the past six months (RRR=0.44, 95% CI=0.21-0.92).

Furthermore, after adjusting for other covariates, the relative risk of getting most syringes from a SEP versus a pharmacy was higher for those injecting one or two times per day in the past six months (versus those injecting less than once per day, RRR=2.88, 95% CI=1.08-7.65) and those injecting three or more times per day in the past six months (versus those injecting less than once per day, RRR=2.92, 95% CI=1.22-7.01). In addition, after adjusting for other covariates, the relative risk of getting most syringes from a SEP versus from another source was lower for whites (versus African-Americans, RRR=0.27, 95% CI=0.09- 0.78), those with a high school education or higher (versus those having less than a high school diploma or a GED, RRR=0.28, 95% CI=0.13-0.64), and those living the furthest away from a SEP (those in the third tertile, compared to those living nearest to a SEP, in the first tertile, RRR=0.20, 95% CI=0.08-0.52).

In summary, this study examined the factors related to where heroin and cocaine injectors obtained most of their new sterile syringes (pharmacies vs. other sources, syringe/needle exchange programs vs. other sources, and syringe exchange programs vs. pharmacies). The study found that IDUs who use SEPs to get their syringes were less likely to live farther away from a SEP and more likely to inject more frequently, compared to those who use other less safe syringes sources or pharmacies. In addition, SEP users and pharmacy users were less likely to share injection equipment compared to those using other less safe sources. SEP users were also more likely to use syringes for fewer hits, compared to those using other sources. Finally, pharmacy users were more likely to be male, white, and have at least a high school diploma (compared to those using other sources). SEP users, compared to pharmacy users, were less likely to be white or have a high school diploma or higher. The findings suggest there are different factors associated with each syringe acquisition source.

This study was one of the few, that we are aware of, to empirically find that distance to a SEP is an important factor in getting syringes from a SEP, such that those living farthest away (between 0.06 and 0.76 miles) were less likely to use SEP (compared to those who use other sources or pharmacies). It is important to note here that all study participants lived within a mile from a SEP site. While several studies note that convenience, especially location, is an important factor in sterile syringe access [50,51], only a few that we are aware of tested this association quantitatively [39-42]. When planning programs for infectious disease prevention, distance to the target population may be an important factor to consider.

Additionally, a main finding of this study was that those who injected more frequently were more likely to get their syringes from SEPs when compared to users of other less safe sources or pharmacies. This finding is consistent with extant research suggesting that SEPs may attract IDUs with more severe drug dependence [30-36]. In the absence of SEPs, those who inject more frequently may not be able to get sterile syringes. Our finding showing those who had been arrested for drug paraphernalia in the past six months were more likely to use SEPs is also consistent with the tenet that SEPs may attract IDUs with more severe dependence. Another explanation for this finding may be that SEP users are more likely to carry syringes with them and get caught and arrested for carrying syringes. At present, it is illegal to possess syringes without a prescription (drug paraphernalia) unless they are obtained from a SEP; in Maryland, SEPs are legal in Baltimore City only [52]. This highlights an important issue: if an injector does not have proof of SEP membership (such as a membership card) or has more than “trace” amounts of drug left in a used syringe that person may be arrested for possessing drug paraphernalia [52]. Given the results of this study, SEPs may be especially important in delivering services, to include referrals to drug treatment and/or health care, to this marginalized and hard-to-reach population.

Consistent with previous studies, this study found that SEP users and pharmacy users were less likely to share injection equipment, compared to users of less safe sources [19,21,22]. SEP users (versus users of other sources) were also less likely to reuse syringes (three or more times) for injection, which is also consistent with previous research [35,53-55]. This study and others support the notion that those using SEPs or pharmacies to acquire sterile syringes (compared to other sources) may engage in fewer high risk injection behaviors. This study offers further support for the claim that giving injectors more access to sterile syringes, through pharmacies or SEPs, may lower their risk of injection equipment sharing and reuse of contaminated syringes, thereby lowering their risk of contracting and transmitting infectious diseases.

Study results also support findings that pharmacies and SEPs serve different populations of IDUs. In this study, IDUs who were male, white, and had more education were more likely to use pharmacies to get syringes, compared to other sources. SEP users, in comparison to pharmacy users, were also less likely to be white or have a high school diploma. There are several empirical studies which suggest that pharmacy users tend to be less disadvantaged than those using SEPs [38] and that class and ethnic differences may influence the sale of sterile syringes to IDUs in pharmacies [15,37]. Although it is legal to purchase syringes without a prescription in Baltimore City, regulations give discretion to pharmacists to restrict the sales of syringes to medical purposes only [52]. Some pharmacists see HIV prevention as a medical purpose while others do not [56].

The present study had some limitations that should be considered. The study’s cross-sectional design precludes causal claims. Also, the study sample was recruited from one urban area, Baltimore, MD and participants were not randomly selected; therefore, they may not be representative of injection drug users in other areas. Additionally, the outcome was measured by participants’ self-report of where they obtained most of their new syringes in the past six months. Some of the participants may have obtained their syringes from multiple sources which may introduce misclassification bias. Further, studies such as this one using self-report measures are subject to biases, such as recall and social desirability. However, the query time period was recent, during the past 6 months, and a review by Darke and colleagues (1998) [57] suggests that self-report data from IDUs are valid and reliable. Only 16 (5.3%) participants reported distributive sharing of syringes in the past six months (selling, returning, or giving a syringe away after using it); therefore, this variable was not included as an independent variable in the analyses. Also, this study was unable to examine how distance to a pharmacy may affect where IDUs get their syringes.

Overall, the current study addressed a gap in the literature by examining the predictors of where drug injectors get their sterile syringe sources (pharmacies, SEPs, and other less safe sources). Study findings indicate that patrons of pharmacies, SEPs, and other sources are different with respect to many characteristics. Findings also suggest that different sources of syringes may complement one another in terms of providing access to sterile syringes. Study findings have implications for the design of targeted intervention programs aimed at preventing the spread of infectious diseases (such as HIV/AIDS and Hepatitis) among injection drug users.

This research was supported by NIDA RO1 Grant DA014498 and NIDA T32 Grant DA007292. The authors have no conflicts of interest to report.

1. Centers for Disease Control and Prevention (CDC). HIV prevalence estimates--United States, 2006. MMWR Morb Mortal Wkly Rep. 2008; 57: 1073-1076.

2. Hall HI, Song R, Rhodes P, Prejean J, An Q, Lee LM, et al. Estimation of HIV incidence in the United States. JAMA. 2008; 300: 520-529.

3. Centers for Disease Control and Prevention (CDC). Advancing HIV prevention: new strategies for a changing epidemic--United States, 2003. MMWR Morb Mortal Wkly Rep. 2003; 52: 329-332.

4. Centers for Disease Control and Prevention. HIV/AIDS Surveillance Report. 1993; 5: 1-19.

5. Ezzati M, Lopez AD, Rodgers A, Vander Hoorn S, Murray CJ. Comparative Risk Assessment Collaborating Group. Selected major risk factors and global and regional burden of disease. Lancet. 2002; 360: 1347-1360.

6. Aceijas C, Stimson GV, Hickman M, Rhodes T. United Nations Reference Group on HIV/AIDS Prevention and Care among IDU in Developing and Transitional Countries. Global overview of injecting drug use and HIV infection among injecting drug users. AIDS. 2004; 18: 2295-2303.

7. Centers for Disease Control and Prevention. Recommendations for prevention and control of hepatitis C virus (HCV) infection and HCVrelated chronic disease. MMWR Recommendations and Reports. 1998; 47: 1-39.

8. National Commission on AIDS. AIDS: an expanding tragedy. The final report of the National Commission on AIDS. Washington, DC: National Commission on AIDS. 1993.

9. Holmberg SD. The estimated prevalence and incidence of HIV in 96 large US metropolitan areas. Am J Public Health. 1996; 86: 642-654.

10. Bourgois P. Theory, method, and power in drug and HIV-prevention research: a participant-observer’s critique. Subst Use Misuse. 1999; 34: 2155-2172.

11. Bourgois P, Lettiere M, Quesada J. Social misery and the sanctions of substance abuse: Confronting HIV risk among homeless heroin addicts in San Francisco. Social Problems. 1997; 44: 155-173.

12. Gayle HD, O’Neill JF, Gust SW, Mata A. HIV prevention bulletin: Medical advice for persons who inject illicit drugs: Centers for Disease Control and Prevention, the National Institute on Drug Abuse of the National Institutes of Health, the Center for Substance Abuse Treatment of the Substance Abuse and Mental Health Services Administration, U.S. Department of Health and Human Services, Public Health Service. 1997.

13. Ingold FR, Ingold S. The effects of the liberalization of syringe sales on the behaviour of intravenous drug users in France. Bull Narc. 1989; 41: 67-81.

14. Gleghorn AA, Jones TS, Doherty MC, Celentano DD, Vlahov D. Acquisition and use of needles and syringes by injecting drug users in Baltimore, Maryland. J Acquir Immune Defic Syndr Hum Retrovirol. 1995; 10: 97-103.

15. Groseclose S, Weinstein B, Jones T, Valleroy L, Fehrs L, Kassler W. Impact of increased legal access to needles and syringes on practices of injecting drug users and police officers - Connecticut, 1992-1993. Journal of Acquired Immune Deficiency Syndromes and Human Retrovirology. 1995; 10: 82-89.

16. Taussig JA, Weinstein B, Burris S, Jones TS. Syringe laws and pharmacy regulations are structural constraints on HIV prevention in the US. AIDS. 2000; 14 Suppl 1: S47-51.

17. Cotten-Oldenburg NU, Carr P, DeBoer JM, Collison EK, Novotny G. Impact of pharmacy-based syringe access on injection practices among injecting drug users in Minnesota, 1998 to 1999. J Acquir Immune Defic Syndr. 2001; 27: 183-192.

18. Vlahov D, Junge B, Brookmeyer R, Cohn S, Riley E, Armenian H, et al. Reductions in high-risk drug use behaviors among participants in the Baltimore needle exchange program. J Acquir Immune Defic Syndr Hum Retrovirol. 1997; 16: 400-406.

19. Vlahov D, Junge B. The role of needle exchange programs in HIV prevention. Public Health Rep. 1998; 113 Suppl 1: 75-80.

20. Bastos FI, Strathdee SA. Evaluating effectiveness of syringe exchange programmes: current issues and future prospects. Soc Sci Med. 2000; 51: 1771-1782.

21. Wodak A, Cooney A. Do needle syringe programs reduce HIV infection among injecting drug users: a comprehensive review of the international evidence. Subst Use Misuse. 2006; 41: 777-813.

22. Coffin P. Syringe availability as HIV prevention: a review of modalities. J Urban Health. 2000; 77: 306-330.

23. Ljungberg B, Christensson B, Tunving K, Andersson B, Landvall B, Lundberg M, et al. HIV prevention among injecting drug users: three years of experience from a syringe exchange program in Sweden. J Acquir Immune Defic Syndr. 1991; 4: 890-895.

24. Burris S, Vernick JS, Ditzler A, Strathdee S. The legality of selling or giving syringes to injection drug users. J Am Pharm Assoc (Wash). 2002; 42: S13-18.

25. Hagan H, Des Jarlais D, Friedman S, Purchase D, Alter M. Reduced risk of Hepatitis B and Hepatitis C among injecting drug users participating in the Tacoma Syringe-Exchange Program. American Journal of Public Health. 1995; 85: 1531-1537.

26. Hagan H, McGough JP, Thiede H, Weiss NS, Hopkins S, Alexander ER. Syringe exchange and risk of infection with hepatitis B and C viruses. Am J Epidemiol. 1999; 149: 203-213.

27. Bluthenthal RN, Kral AH, Gee L, Erringer EA, Edlin BR. The effect of syringe exchange use on high-risk injection drug users: a cohort study. AIDS. 2000; 14: 605-611.

28. Gibson DR, Flynn NM, Perales D. Effectiveness of syringe exchange programs in reducing HIV risk behavior and HIV seroconversion among injecting drug users. AIDS. 2001; 15: 1329-1341.

29. Centers for Disease Control and Prevention (CDC). Update: syringe exchange programs--United States, 2002. MMWR Morb Mortal Wkly Rep. 2005; 54: 673-676.

30. Henderson LA, Vlahov D, Celentano DD, Strathdee SA. Readiness for cessation of drug use among recent attenders and nonattenders of a needle exchange program. J Acquir Immune Defic Syndr. 2003; 32: 229-237.

31. Hahn JA, Vranizan KM, Moss AR. Who uses needle exchange? A study of injection drug users in treatment in San Francisco, 1989-1990. J Acquir Immune Defic Syndr Hum Retrovirol. 1997; 15: 157-164.

32. Mark H, Nanda J, Davis-Vogel A, Navaline H, Scotti R, Wickrema R, et al. Profiles of self-reported HIV-risk behaviors among injection drug users in methadone maintenance treatment, detoxification, and needle exchange programs. Public Health Nursing. 2006; 23: 11-19.

33. Archibald CP, Ofner M, Strathdee SA, Patrick DM, Sutherland D, Rekart ML, et al. Factors associated with frequent needle exchange program attendance in injection drug users in Vancouver, Canada. Journal of Acquired Immune Deficiency Syndromes and Human Retrovirology. 1998; 17: 160-166.

34. Cao W, Treloar C. Comparison of needle and syringe programme attendees and non-attendees from a high drug-using area in Sydney, New South Wales. Drug Alcohol Rev. 2006; 25: 439-444.

35. Bailey SL, Huo D, Garfein RS, Ouellet LJ. The use of needle exchange by young injection drug users. J Acquir Immune Defic Syndr. 2003; 34: 67-70.

36. Schechter MT, Strathdee SA, Cornelisse PG, Currie S, Patrick DM, Rekart ML, et al. Do needle exchange programmes increase the spread of HIV among injection drug users?: an investigation of the Vancouver outbreak. AIDS. 1999; 13: F45-51.

37. Singer M, Baer HA, Scott G, Horowitz S, Weinstein B. Pharmacy access to syringes among injecting drug users: follow-up findings from Hartford, Connecticut. Public Health Rep. 1998; 113 Suppl 1: 81-89.

38. Wodak A, Cooney A. Effectiveness of sterile needle and syringe programming in reducing HIV/AIDS among injecting drug users. Geneva, Switzerland: World Health Organization. 2004.

39. Rockwell R, Des Jarlais DC, Friedman SR, Perlis TE, Paone D. Geographic proximity, policy and utilization of syringe exchange programmes. AIDS Care. 1999; 11: 437-442.

40. Williams CT, Metzger DS. Race and distance effects on regular syringe exchange program use and injection risks: a geobehavioral analysis. Am J Public Health. 2010; 100: 1068-1074.

41. Cooper H, Des Jarlais D, Ross Z, Tempalski B, Bossak BH, Friedman SR. Spatial access to sterile syringes and the odds of injecting with an unsterile syringe among injectors: a longitudinal multilevel study. J Urban Health. 2012; 89: 678-696.

42. Cooper HL, Des Jarlais D, Tempalski B, Bossak BH, Ross Z, Friedman SR. Drug-Related Arrest Rates and Spatial Access to Syringe Exchange Programs in New York City Health Districts: Combined Effects on the Risk of Injection-Related Infections among Injectors. Health Place. 2012; 18: 218-228.

43. Mitchell MM, Severtson SG, Latimer WW. Interaction of cognitive performance and knowing someone who has died from AIDS on HIV risk behaviors. AIDS Educ Prev. 2007; 19: 289-297.

44. Vlahov D, Anthony JC, Celentano D, Solomon L, Chowdhury N. Trends of HIV-1 risk reduction among initiates into intravenous drug use 1982-1987. Am J Drug Alcohol Abuse. 1991; 17: 39-48.

45. Baltimore Substance Abuse Systems. Needle exchange services. 2007.

46. Beyer H. Hawth’s Analysis Tools for ArcGIS. 2004.

47. van Buuren S, Boshuizen HC, Knook DL. Multiple imputation of missing blood pressure covariates in survival analysis. Stat Med. 1999; 18: 681-694.

48. Royston P. Multiple imputation of missing values. The Stata Journal. 2004; 4: 227-241.

49. Mehrotra DV, Li X, Liu J, Lu K. Analysis of longitudinal clinical trials with missing data using multiple imputation in conjunction with robust regression. Biometrics. 2012; 68: 1250-1259.

50. Vlahov D, Ryan C, Solomon L, Cohn S, Holt MR, Akhter MN. A pilot syringe exchange program in Washington, DC. Am J Public Health. 1994; 84: 303-304.

51. Sarang A, Rhodes T, Platt L. Access to syringes in three Russian cities: implications for syringe distribution and coverage. Int J Drug Policy. 2008; 19 Suppl 1: S25-36.

52. Burris S, Strathdee S, Vernick J. Syringe Access Law in the United States: A State of the Art Assessment of Law and Policy. Baltimore, MD: Center for Law and the Public’s Health, Johns Hopkins Bloomberg School of Public Health, Georgetown University. 2002.

53. Huo D, Ouellet LJ. Needle exchange and injection-related risk behaviors in Chicago: a longitudinal study. J Acquir Immune Defic Syndr. 2007; 45: 108-114.

54. Bravo MJ, Royuela L, Barrio G, Brugal MT, Domingo A, de la Fuente L. Itínere Project Group. Access to sterile syringes among young drug injectors in Madrid and Barcelona and its association with risk behaviour. Gac Sanit. 2008; 22: 128-132.

55. Ouellet L, Huo D, Bailey SL. HIV risk practices among needle exchange users and nonusers in Chicago. J Acquir Immune Defic Syndr. 2004; 37: 1187-1196.

56. Golub ET, Bareta JC, Mehta SH, McCall LD, Vlahov D, Strathdee SA. Correlates of unsafe syringe acquisition and disposal among injection drug users in Baltimore, Maryland. Subst Use Misuse. 2005; 40: 1751- 1764.

57. Darke S. Self-report among injecting drug users: a review. Drug Alcohol Depend. 1998; 51: 253-263.