Background: Limited information suggests that men who have sex with men (MSM) are informally obtaining antiretroviral medication (ARVs) and using them for HIV pre-exposure prophylaxis (PrEP). Methods: Data are drawn from an on-going study examining the use of non-prescribed ARVs for PrEP. To date, 24 qualitative interviews have been conducted with HIV-negative, substance-using MSM living in Miami, Florida, USA. Data are presented from two participants who reported HIV seroconversion while using non-prescribed ARVs for PrEP. Results: Preliminary data indicate that some young MSM: (i) lack awareness of and accurate information about the efficacious use of PrEP; (ii) obtain non-prescribed ARVs from HIV-positive sex partners and use these medications for PrEP in a way that does not provide adequate protection against HIV infection or cohere with established guidelines; and (iii) engage in multiple HIV transmission risk behaviours, including condomless anal sex and injection drug use. Conclusions: The informal, non-prescribed and non-medically supervised use of ARVs for HIV prevention has the potential to undermine the protective benefits of PrEP and leave men unprotected against HIV transmission and at risk for ARV resistance.