Adverse drug reactions are a major cause of hospital admission and in-hospital morbidity. Antimicrobial agents are one of the most commonly prescribed groups of drugs leading to adverse drug reactions. But, limited data is available from hospital settings in our country. Hence, it is important to understand the characteristics of adverse reactions associated with their use. This study was planned to detect the adverse drug reactions through spontaneous reporting system and analyse them using standard assessment scales. It is an observational, prospective study performed in patients attending outpatient department, in-patient department and casualty of tertiary care hospital from 1st June 2005 to 31st May 2009. Patients were examined for adverse drug reactions by spontaneous adverse drug reactions reporting system. Among 2639 total adverse drug reactions reported, 1315 (49.82%) were due to antimicrobial agents. The agents most frequently associated with adverse drug reactions were cotrimoxazole (19.54%), ampicillin (18.25%), rifampicin (6.6%) and azithromycin (5.3%). It is observed that commonly used drugs like antimicrobial agents were causing maximum adverse drug reactions. This study provides a database of adverse drug reactions due to common drugs, which will help clinicians for optimum and safe use of these drugs.