OBJECTIVE: The objective of this study was to identify Drug-Drug Interaction among the inpatient admitted in ICU and the Medical Ward of a tertiary care teaching hospital in India. METHOD: This study was a prospective observational study conducted over a period of 6 months from July 2016 to January 2017 among inpatients of ICU and Medical Ward. The prescriptions having 2 or more drug and where DDIs was suspected were selected. The drugs in the prescription were analyzed through drug interaction checker software. The DDIs were classified based on the severity of Interactions, mechanism of interaction, the relation of disease and drug interaction, and the total number of drug prescribed were determined. RESULT: There was a total of 225 patients in our study. Out of 225(100%) total patients who had drug-drug interactions, There were a total of 72(32%) patients in ICU and a total of 153(68%) patient in the Medical Ward. An average of 7.625 drugs per prescription was prescribed in ICU and an average of 5.70 drugs per prescription were prescribed in Medical Ward. There were 1333 drug-drug interaction. Among them, 523 DDIs were in ICU and 810 DDIs were in the Medical Ward. A severity assessment showed that in both ICU and Medical ward, majority of interaction were moderate followed by minor interaction. In ICU (57.93%) and in the Medical Ward (62.96%) were moderate drug interaction respectively. On assessing the mechanism of DDIs, pharmacodynamic interaction in ICU were (34.99%) which was higher than pharmacokinetic interaction which was (33.46%) followed by unknown mechanism (31.54%). Whereas in Medical ward most of the interaction was pharmacokinetic Interaction (45.55%) followed by an unknown mechanism (30.74%) and pharmacodynamic interaction (23.70%). Finding of this study showed that cardiovascular disease had (24.28%) and respiratory disease had (18.54%) DDIs in ICU. Similarly, in the Medical Ward cardiovascular disease had (36.79%) and respiratory disease had (23.33%) DDIs. This study result showed that as the number of drug increases in a prescription, the number of DDIs also increases. The management required for DDIs in the study was Dosage adjustment which was 16% followed by monitoring drug level which was 15%. CONCLUSION: A significant number of drug interactions were seen in the prescription of the inpatient of ICU and Medical ward. The most prevailing interaction was Moderate interaction which was the highest number followed by minor interaction. Polypharmacy was the major factor responsible for DDIs. This study highlights the need for intense monitoring of patients to help detect and prevent them from serious health hazards associated with DDIs.