Clinical efficacy of antiretroviral treatment requires drugs to maintain a plasma concentration over MIC90. One of the facts that can prevent drugs from reaching and maintaining required concentrations are pharmacological drug interactions.The aim of this study is to identify the main interactions between antiretroviral medicines and concomitant treatment in a HIV positive group of patients in a general hospital. Clinical significance and the way to avoid or minimize these interactions is indicated.from the computerized records of dispensation the Pharmacy Service and from the reports of the internal medicine deparment, demographic and epidemiological data were collected, as well as previous and current antirretroviral treatment and concomitant treatment. Interactions were identified after the Guide of Pharmacological Interactions for HIV.the number of patients analysed was 189. Of these, in 153 (81%) 466 interactions were detected (2.46 interactions/patient). The classification according to its clinical significance was the following: no clinical relevancy (52.8%), potential interactions (45.5%) and co-administered drugs contraindicated or not recommended (1.7%, nA masculine = 8). This last one corresponded to the following medications: efavirenz/terfenadine, nelfinavir/terfenadine, ritonavir/diazepam and zidovudine/ribavirin (five cases). Zidovudine was the one with the highest number of interactions detected (129 patients) followed by indinavir while the co-administered drug was cotrimoxazole (93 cases). The remaining interactions detected for all the anti-retrovirals are shown. We conclude that thought the number of drug interactions is high, the number of contraindicated or not recommended ones is low. Finally the therapeutic alternatives for those interactions detected that show the highest clinical relevancy are described. General procedures to identify and prevent them are also described.We conclude that though the number of drug interactions is high, the number of contraindicated or not recommended ones is low. Finally the therapeutic alternatives for those interactions detected that show the highest clinical relevancy are described. General procedures to identify and prevent them are also described.