Background: An important cause of treatment failure to antiretroviral therapy (ART) is the potential interaction between the antiretroviral (ARV) drugs and co-prescribed drugs used concomitantly for the treatment of opportunistic infections and co-morbid ailments in HIV-infected patients. Objectives: The study evaluated potential clinically significant drug interactions (CSDIs) occurring between recommended ART regimens and their co-prescribed non-antiretroviral drugs (CPD) Method: This study was carried out in a large HIV treatment centre (APIN clinic) in a Nigerian teaching hospital, in Lagos Nigeria, caring for over 20,000 registered patients. Electronic Medical Records (EMR) of 500 patients who received treatment between 2005 and 2015, were selected using systematic random sampling, reviewed retrospectively, and evaluated for potential CSDIs using Liverpool HIV Pharmacology Database and other similar databases. Results: Majority of patients, 421 (84%) were at risk of CSDIs, of which 410, (82%) were moderate and frequently involved co-trimoxazole + zidovudine (or stavudine) /lamivudine (386, 77.2%) and NNRTIs or PIs + artemisinin-based combination therapies (ACTs) [296, 59.2%]. Age (p=0.131), sex (p=0.316) and baseline CD4+ cell counts (p>0.05) were not significantly associated with CSDIs. The interactions, however, were significantly associated with the development of antiretroviral treatment failure (p <0.001) which occurred in nearly a third (139; 27.8%) of the patients. Conclusion: There is a high prevalence of CSDIs between ART and CPDs most of which were categorized as moderate. Further studies are required to evaluate the pharmacokinetic and clinical relevance of these interactions.