BACKGROUND: The introduction of antiretroviral therapy has caused a remarkable decrease in the occurrence of diseases and mortality among HIV-positive patients, while this success has not been achieved among injection addicts due to a low adherence to antiretroviral medicine. This study aims at clarifying the important factors affecting adherence to treatment in addicts suffering from HIV. MATERIALS AND METHODS: In this qualitative research, data were gathered through in-depth interviews and field notes, and were interpreted through content analysis in the form of constant comparison. The participants were 16 drug addicts living with HIV/AIDS. Most of them had records of imprisonment and were receiving Highly Active Antiretroviral Therapy (HAART) drug treatments in the AIDS center of Imam Khomeini Hospital complex, affiliated to Tehran University of Medical Sciences. Sampling was started in a purposive method and was continued until data were saturated. RESULTS: Four main categories including psychological reactions, contradictory beliefs, perceived support, and individual and environmental barriers were extracted from the data, each having some sub-categories. CONCLUSIONS: The obtained results indicated that adherence to the treatment of HIV is not constant and mono-dimensional, but is a function of different factors. Hence, an individual having feeble adherence in a specific time and under specific circumstances may show desirable adherence under a different circumstance. Thus, treatment of addicts living with HIV/AIDS requires physical, psychological, and social attention along with drug treatments.