Objectives The main goal was to assess the reasons for antiretroviral therapy (ART) change in patients with HIV in a hospital setting in routine clinical practice. The economic impact of ART modification was also analysed.

Methods Patients with HIV who changed their ART between 24 November and 24 December 2014 were registered. Length of initial therapy, type of ART before and after therapy modification, and reasons for the ART change were analysed. To assess the economic impact, antiretroviral drug costs at the time of the study were recorded.

Results Of a cohort of 3850 patients with HIV, 1976 attended for pharmaceutical care consultation at Hospital Universitario La Paz during the study period. Ninety-two patients (4.7%) had their ART modified. The median length of the previous therapy was 26 months (range 1–144). The most common initial therapy regimen was 2 nucleoside reverse transcriptase inhibitors (NRTI)+1 non-nucleoside reverse transcriptase inhibitor (NNRTI) (29.4%), and the most common one after modification was 2 NRTI+1 integrase strand transfer inhibitor (INSTI) (40.2%). Forty-three modifications were made because of toxicity and adverse effects (46.7%), 25 because of therapy simplification (27.2%), 16 because of treatment failure (17.4%), and 8 because of drug–drug interactions (8.7%). ART costs increased by a mean of €14 (SD €216; range −€528 to +€831) per month per patient after therapy modification at the time of study.

Conclusions Toxicity and adverse effects were the most common reason for ART alteration in patients with HIV in routine clinical practice in a hospital setting. Better knowledge about factors that motivate these changes may contribute to decreased toxicity and increased treatment success. ART modification had a variable but not very substantial economic impact.