The benefits of HIV treatment (Highly Active Antiretroviral Therapy [HAART]) have been less apparent in injecting drug users (IDUs), most probably as a result of poor adherence to treatment. We explored factors related to HIV treatment adherence as reported by 23 IDU-HIV patients and nine health professionals from healthcare services in Alicante and Valencia, Spain. We carried out a qualitative study based on personal interviews. Health professionals reported the lack of coordination among hospital services and difficulties in accessibility to nonspecialized services for IDU-HIV patients as relevant factors for treatment adherence. Their perception of a patient’s likelihood of treatment adherence was also considered to influence the decision to prescribe HAART. A better treatment adherence was reported by those IDU-HIV patients with a good doctor–patient relationship and by women with family responsibilities. Patients considered the side effects of HIV treatment, the lack of social support, and the active use of recreational drugs as relevant factors to explain incompliance. Interventions and training of health providers should be aimed at the reduction of barriers in patient–provider communication and the overcoming of stereotypes, thus avoiding discriminatory attitudes in treatment in this vulnerable population.