Objectives The aim of the present work was to determine the influence of co-medication on the incidence of clinically relevant interactions, as well as their effect on the clinical condition, of patients with HIV who are on antiretroviral treatment and have a regular follow-up at a pharmaceutical care facility specialising in viral diseases.

Methods An open-label, single centre prospective study was conducted at a hospital from January to December 2010. Inclusion criteria were age > 18 years, regular monitoring and active antiretroviral therapy. Disease monitoring was carried out by plasma viral load and CD4 T cell count measurements.

Results Drug prescriptions were analysed for 468 patients. The mean patient age was 45 years. Overall, 20% of the patients had a viral load <50 copies/ml, 60% had co-medication and the mean number of drugs prescribed per patient was 2.70 (SD±2.35). A total of 2550 drug interactions were noted; 1447 HIV–HIV drug interactions and 1133 HIV–non-HIV drug interactions. Gastric acid secretion inhibitors, methadone, antidepressant and antipsychotics were the drug types with the highest frequency of interactions. Patients with CD4 T cell counts ≤250 cells/µl showed more drug interactions than patients with values >250 cells/µl. Clinically significant drug interactions were lower in adherent patients than in non-adherent patients. In the multivariate analysis, risk factors associated with clinically significant drug interactions were co-medication, treatment with a protease inhibitor and backbone ‘others’ group.

Conclusions Co-medication increases the number of clinically relevant drug interactions in patients with HIV under active antiretroviral treatment, resulting in lower levels of CD4 T cell counts and clinical worsening.