Illicit use of methadone was not uncommon in this population of street-recruited drug users in NYC. Approximately 21.8% of NIDUs and 52.6% of IDUs in this sample of street-recruited drug users had used illicit methadone in their lifetime and 11.8 and 34.1% had used within the last six months, respectively. A previous study had estimated a 21.3% past month prevalence for illicit methadone among Puerto Rican IDUs in East Harlem, New York [25]. In Australia, the lifetime and six month prevalence of injected methadone was 18.4% and 11.0%, respectively [6]. Our data did not distinguish between route of administration and the Australian study did not report on non-injected illicit methadone use, thus comparison between the estimates is limited. In Montréal, the illicit methadone prevalence was 42.1% among those who preferred heroin [5]. Although we did not ask about drug preference, frequency of use can serve as a proxy measure. Among the NIDUs and IDUs, approximately 38.7% of daily heroin users and 19.7% of less than daily heroin users also used illicit methadone. Differences in the prevalence of illicit methadone use among these studies are probably due, in part, to differences in sampling schemes and differences in measurement. However, methadone prescribing policies differ between countries and also play an important role in MMTP and illicit methadone availability.

Our study revealed that illicit methadone users engaged in riskier behavior than those who did not use street methadone, as evidenced by the higher prevalence of overdose experiences, polysubstance use and risky sex partners among NIDU illicit methadone users. Humeniuk and colleagues have suggested that methadone injectors are riskier than those who do not inject methadone, with methadone injectors in their population being more likely to have overdosed, used heroin by themselves and be a polysubstance user [6]. Further, we observed that IDUs who used street methadone reported riskier injection practices compared to IDUs who did not. Specifically, they were more likely to attend a shooting gallery and buy, rent or borrow injection equipment at a shooting gallery as well as share needles and other injection paraphernalia (i.e. cookers, cotton and rinse water). This is a novel finding. Hopwood and colleagues reported that methadone syrup injecting in New South Wales was not associated with sharing injection equipment, but it was associated with reuse of methadone injecting equipment [26].

Predictably, recent illicit methadone use was associated with heroin dependence among NIDUs and daily heroin use among IDUs. It is likely that dependence and daily use are measuring the same underlying construct of severity of use. Coupled with the data demonstrating relatively low frequency of use and later age of onset as compared to other both heroin and injection drug use, these findings suggest that illicit methadone use is likely not a primary drug of choice. This finding parallels that of Lauzon and colleagues in Montréal [5], who reported a later age of onset for illicit methadone use as compared to heroin, cocaine and a variety of other substances. The association between heroin dependence, daily heroin use and illicit methadone use suggests that methadone is used ancillary to heroin.

In the multivariate model for NIDUs, having a heroin using partner in the last two months was associated a more than two-fold increase in the likelihood of illicit methadone use. One possible explanation is that the sex partners are also drug use partners, pooling resources to purchase drugs. This would then suggest that having a heroin using sex partner may be related to more severe heroin use. Less than daily methamphetamine use was associated with a more than five-fold increase in the likelihood of illicit methadone use. Considering the low prevalence of methamphetamine use in this population, this association may be spurious. Little is known about methamphetamine use in NYC, although several recent studies have examined its use among gay and bisexual men [27-29]. These findings need further exploration.

Illicit methadone use among IDUs was associated with a significant increase in the likelihood of sharing injection paraphernalia such as cookers, cottons and rinse water, even after adjusting for daily heroin use, suggesting that street methadone use may be part of a larger profile of risky drug use behavior. Surprisingly, IDU illicit methadone users had a significantly lower likelihood of recent detoxification treatment compared to non users. One possible explanation is that detoxification may be perceived as ineffective or undesirable to chronic heroin users. Another explanation may be that IDUs are using street methadone to detox themselves.

As with any study, this study is subject to several limitations. The study sample was limited to users of heroin, crack and cocaine and specifically recent initiates to injection drug users and those who had never injected drugs. Other groups that may use illicit methadone including former and current pain patients, individuals who abuse prescription opioids, youth and other populations were not included. Correlates of illicit methadone use among those who are not heroin users and among those whose drug of choice is methadone are likely different. Further, the extent to which these findings are generalizable to other settings with different demographic profiles is unknown. Our population was relatively young and had few White drug users and Black injectors.

Availability and popularity of specific substances vary by region and the extent to which heroin is a primary drug of choice can vary widely [30,31]. Opioid analgesic sales per 100,000 population also vary widely by state [20]. Bourgois [32] has pointed out that methadone prescribing philosophies can differ between cities such that availability of methadone may be different between cities. We did not have data on route of administration of street methadone and therefore cannot look specifically at methadone injection. We do not think that injection is a common route of administration; Humeniuk et al. [6] reported that less than 20% in a study of heroin users in Australia reported lifetime methadone injection.

These data do not provide a complete picture of illicit methadone use and thus are limited. Two important questions remain unanswered. First, the reasons for illicit methadone use were not investigated. There are several possible reasons that individuals may use illicit methadone, including unavailability of heroin, underdosing in methadone programs and/or pain management, and experimentation prior to entry into a methadone program. In Australia, one study reported that 58% of methadone injectors preferred injection because it provided quicker relief of opiate withdrawal symptoms than drinking the syrup [26], suggesting that underdosing may be an important consideration. A study of U.S. methadone maintenance facilities found that approximately one-third provided dose under the recommended level [33]. Some proportion of illicit methadone use might then be attributed to attempts at self-medication either by supplementing MMTP, reducing dependence on heroin without the use of a formal treatment program, or preventing withdrawal symptoms when heroin is otherwise not available as has been suggested in earlier studies [7,34]. A recent study of patients in a German detoxification ward reported that approximately one-third reported using diverted opiods as an attempt at self-detoxification, when a dose of prescribed methadone had been missed, or as a transition before entering methadone maintenance treatment [35]. It has also been suggested that low availability of methadone treatment slots may be an important factor in the creation of an illicit street-level market for methadone [5]. Although bivariate analyses demonstrated a relationship between MMTP and illicit methadone use among NIDUs, this association was not significant in the multivariate analysis. Only 30.4% of illicit methadone users were recently in MMTP, suggesting that illicit methadone use may not simply be a result of underdosing of MMTP patients or personal diversion of medication. From these data, it is unclear if the remaining 67.6% of illicit methadone users are former MMTP patients. Second, the illicit methadone sources of participants in this study remain unknown. Several avenues for acquisition are likely available, including methadone available due to theft from hospitals or pharmacies, diversion from maintenance programs, or diversion from pain management prescriptions.