OBJECTIVE Patients in the intensive care unit (ICU) have multiorgan dysfunction as well as altered pharmacokinetic parameters. Hence they are susceptible to adverse drug reactions (ADRs). The objective of the study is to assess the characteristics of ADRs among inpatients in the medical ICU and to compare the same with patients who have not experienced ADRs. METHODS Prospective, observational study for a period of 1 year in medical ICU of a tertiary care hospital. Relevant data of patients with ADRS were analysed. Characteristics of patients with and without ADRs were compared. RESULTS Of 728 patients admitted in medical ICU, 222 (28.4%) had ADRs. Multiple ADRs (38.7%) implicated by the same drug and serious ADRs (37%) were noticed. Renal/electrolyte system (21%) was most commonly involved. Clinical spectrum included acute renal failure (ARF, 11.4%), hepatic injuries (5.4%), haematological dysfunction (4.2%), seizures (3.3%), upper gastrointestinal bleed (3.3%) and cutaneous ADRs (3.3%). Antimicrobials (27%) were the commonly implicated drug class. The most commonly implicated drug was furosemide (6.8%). Infrequently reported ADRs included azithromycin-induced erythema multiforme, leflunamide-induced erythema multiforme and vasculitis, ceftazidime-induced seizures and ceftriaxone-induced hepatitis. Co-morbidity, polypharmacy and duration of stay were significantly higher in patients with ADRs compared to those who have not experienced ADRs. Three patients died. CONCLUSIONS High incidence of serious and multiple ADRs noticed. A wide clinical spectrum of ADRs and infrequently reported ADRs to newer drugs were also observed.