Irrational use of medicines exemplified by polypharmacy, over use of injections and antimicrobials amongst others have resulted in increased morbidity and mortality for patients as well as influencing the pattern of adverse drug reactions and the cost of treatment incurred by patients and the community. The indicators developed by the WHO for objective evaluations of rational medicine use can be deployed to characterize current medication use patterns in our setting. The aim of this study is to evaluate the pattern of use of medications using the WHO prescribing indicators and to profile the pattern of adverse drug reactions as well as their cost implications in a tertiary health care setting with a view to developing parameters to serve as basis for reference purposes, policy making and interventions, in a bid to attain safe therapeutic patient care. Methods: This was a descriptive prospective study in which in-patients admitted into medical wards of a tertiary hospital were followed up using a data collection form modified from WHO prescribing indicator form. Patients were evaluated on specific days during admission and were monitored for adverse drug reactions occurrence. Cost of therapy was evaluated using hospital pharmacy price list. SPSS version 13 software was used for analysis with results being expressed as means ± S.D, percentages or median values where necessary. Means were compared using T- test and association of proportions were evaluated using chi – squared test with level of significance set at 95%. Results: Five hundred and seven patients (507) were evaluated over a period of 8 months, 269 males and 238 females. The mean age of studied population was 48.9 ± 17.8 years (range- 17 to 89 years). The most prevalent disease entities were malaria, HIV/AIDS, diabetes mellitus and hypertension occurring in 18.9%, 17.2%, 17% and 16.8% of patients respectively. „Medicines used for alimentary tract and metabolism‟ (ATC class A), was the most frequently prescribed class of medicines, constituting 25.6% of all prescriptions. The mean duration of admission was 11.9 ± 11.3 days, while the mean number of medicines used per admission was 9.1 ± 3.8. The percentage of medicines prescribed by generic name was 85.6% while the percentage prescribed from the essential medicines list was 88.1%. The percentage of admissions with injectables prescribed was 89.3%, percentage with antimicrobials prescribed was 61.9% while 18.9% admissions had antimalarials prescribed. An average cost of N16,993.61 ± N 25415.71 ($130.72 ± $195.51) was spent per admission while an average of N 3169.96 ± N 6348.77 ($24.38 ± $48.84) was spent on treating adverse drug reactions. Patients admitted due to Adverse drug reactions (ADRs) were 3.6% while in 6.5%, ADRs occurred during admission, giving an ADR prevalence of 10.1%. Identified risk factors for ADRs were age ≥ 65 years (chi square = 10.152, P = 0.001), number of medicines patient was on (chi square = 11.891, P = 0.018) and number of co-morbidities (chi square = 21.962, P = 0.000). Medicines most often implicated in ADRs were insulin (27.5%) and NSAIDs (19.6%) while the organs/systems most affected were the nervous system (33.3%) and the gastrointestinal system (21.6%) in the form of hypoglycemia and upper gastrointestinal bleeding respectively. The percentage of the total medicine cost spent on treating ADRs was 1.9%. Conclusion: The WHO medicine use indicators serve as a tool for monitoring medicine use and have been successfully adapted to in-patients in this study. Adverse drug reactions with an observed incidence of 6.5% and prevalence of 10.1% are quite common and represent a sizeable burden in terms of morbidities and cost to patients.