Clopidogrel, as an adjunct to aspirin, has been recommended for patients undergoing percutaneous coronary intervention (PCI) to reduce adverse cardiovascular events. Clopidogrel needs activation via the cytochromes P450 enzymes. Concomitant use of clopidogrel with drugs that utilize the same enzymatic pathway may interfere with its conversion to an active form. This study aims to assess the impact of concomitant drug therapy on clopidogrel efficacy in patients undergoing PCI and to identify the association between the use of those drugs and the risk of future adverse cardiac events. A total of 126 medical records for patients followed at King Fahad Specialist Hospital, and King Saud Hospital in Qassim, Saudi Arabia, were reviewed. Patients were divided into six groups: First group clopidogrel, second to fourth group clopidogrel plus proton pump inhibitors (PPIs)/calcium channel blockers (CCBs)/statins, fifth group clopidogrel with either PPIs, CCBs, statins, and sixth group clopidogrel plus PPIs, CCBs, and statins. Descriptive statistics were performed to detect the correlation between the concomitant drug used and MACEs. Out of the total number, 58 patients had MACEs. The correlation between MACEs and medications used in the groups of clopidogrel with PPIs, statins, or combinations of PPIs with CCBs and statins, or combination of two drugs had a MACEs rate of 12% (P= 0.027), 17.2% (P= 0.0041), 19% (P= 0.0021), 43.1% (P= 0.0001) respectively in comparison to 3.45% in the clopidogrel group. Therefore, concomitant use of clopidogrel with PPIs, statins, or combinations of PPIs with CCBs and statins, or a combination of two drugs is associated with a higher risk of MACEs. This suggests that these medications may attenuate the antiplatelet effect clopidogrel. Prospective randomized studies are needed to provide firm evidence for this interaction.