Background The number of HIV infected patients with other comorbidities is growing due to increased life expectancy. So many patients have very complex therapeutic regimens that could interfere with adherence. Purpose To determine the effect of the complexity of the drug regimen on the adherence to antiretroviral treatment (ART) and lipid-lowering treatment (LLT). Materials and methods We conducted a single-centre, retrospective study. We included HIV infected patients with ART and treatment for dyslipidaemia between January–June 2013. The dependent variable was the adherence (ART and LLT) and the independent variables were: sex, age, route of HIV transmission, HCV coinfection, alcohol consumption or illegal drug abuse, psychiatric disease and complexity of the drug regimen. Adherence was determined through pharmacy dispensing records. Patients were considered adherent when they took ≥90% of prescribed ART and LLT in the last 3 months. Drug regimen complexity was determined through the tool “medication regimen complexity index” (MRCI) developed by McDonald et al1. To determine the variables associated with adherence, we performed a univariate logistic regression analysis. Results We included 55 patients in the study (82% men, mean age 55 years). Sexual was the main route of HIV transmission (40%). 52.7% were co-infected with HCV, and 15% of patients used alcohol or illegal drugs. Atorvastatin was the LLT most frequently prescribed. 82% of patients were adherent to ART, but only 69% presented undetectable HIV-RNA. On the other hand, 51% of patients were adherent to LLT. MRCI was not a predictive factor for non-adherence. Alcohol consumption or illegal drug abuse was the only variable that showed statistically significant relationships with the non-adherence to ART (p = 0.013). Adherence to ART in this group of patients was 40% vs. 90.9% in the other group (not consuming alcohol or illegal drugs). Conclusions In this study the complexity of the drug regimen was not a predictive factor for adherence in HIV infected patients. Alcohol consumption or illegal drug abuse could lead to a lack of adherence. Hospital pharmacists play a key role in adherence to ART and this study showed a high adherence to ART. However, at present many patients have other prescription drugs for other comorbidities. In this study the adherence to LLT is low. Therefore, hospital pharmacists should try to ensure adherence to all the medicines and not only to ART. Reference McDonald MV, Peng TR, Sridharan S, Foust JB, Kogan P, Pezzin LE, Feldman PH. Automating the medication regimen complexity index. J Am Med Inform Assoc 2013. 1;20:499-505. No conflict of interest.