Achieving blood pressure goals usually requires two or more antihypertensive agents (AHAs); however; increasing the number of AHAs in a regimen may lead to even more adverse drug reactions (ADRs). The objective of the study is to describe the common systems involved in adverse drug reactions associated with antihypertensive therapy and categorise them according to Naranjo algorithm.  METHODS: We studied 382 adult (18 years or older) hypertensive patients on anti-hypertensive treatment at department of Cardiology at a tertiary referral center in Central Nepal from July to September 2014. Naranjo algorithm was used to categorise the causality relationship into possible, probable, definite and doubtful. RESULTS: : Out of 382 patients in the study period, 140 (36.65%) were receiving monotherapy and 242 (63.35%) multiple drug therapy. Among the total 67 (17.54%) ADR cases reported, Calcium channel blockers contributed to 22 (32.84%) ADRs followed by Angiotensin converting enzyme inhibitors in 17 (25.38%), Angiotensin receptor blockers (13 or 19.4%), diuretic (9 or 13.44%) and Beta adrenergic antagonists to (6 or 8.95%) cases. Cardiovascular system (40 or 59.70%) was the most affected followed by Central Nervous System (16 or 23.88%) and Respiratory and dermatological system each in 11 (16.42%) cases. Only nine (13.4%) ADRs were definite, 16 (23.9%) probable, 39 (58.2%) possible and three (4.5%) cases were categorized as doubtful as per Naranjo algorithm.  CONCLUSIONS: Cardiovascular system was the most common system affected by the ADR of antihypertensive agents and only a few could be categorised as definite ADRs based on Naranjo algorithm. ADR monitoring needs to be done in hospital settings continuously so that untoward effect caused by antihypertensive agents can be identified and documented.