BACKGROUND: To explore the use of illicit drugs by people living with HIV (PLHIV) taking antiretroviral therapy (ART) and their relationship with variables relevant to the management of HIV infection, such as knowledge and beliefs about drug-drug interactions (DDIs), ART adherence, quality of life (QoL), and use of health-care resources. METHODS: 21 PLHIV in Spain who concomitantly took illicit drugs and ART participated in this qualitative study. Eight experts collaborated in the design of the semi-structured interview guide which explored the following topics: illicit drug use, knowledge and beliefs about DDIs and their impact on ART adherence, the effects of using illicit drugs on health, QoL, and use of health-care resources. Four of those experts, who were PLHIV and members of the executive boards of non-government organizations (NGOs) from four Spanish regions, recruited the participants through their NGOs and carried out the face-to-face interviews. Content analysis of the qualitative data was conducted with the support of the MAXQDA 12 program. RESULTS: Participants were mainly men (85.7%) and only 14.3% of them were heterosexual. Content analysis showed that the most frequently consumed illicit drugs were poppers, cocaine, and cannabis. Participants were polydrug users and this was, in many cases, prior to HIV diagnosis. Most participants presented theoretical potential moderate DDIs that would require monitoring. More than three quarters of them were not aware of these DDIs. Participants reported interactive toxicity beliefs that lead to intentional nonadherence behaviors. In most cases (n = 17), the participant's doctor knew about their drug use, however only six of them had had an open dialogue with their physician about it. Illicit drug use led to some health-related problems, mainly sexually transmitted infections. A positive QoL's self-perception was found among several participants that used recreational illicit drugs. CONCLUSIONS: Adequate information about DDIs and clues about how to manage ART when PLHIV are using illicit drugs could reduce the negative effects of such interactions and improve ART adherence and QoL.