ObjectiveTo assess the impact of pharmacists’ participation on the frequency and depth of medication history information documented in a developing setting like NigeriaMethodThe study consisted of two phases. The first phase was a baseline cross-sectional assessment of the frequency and depth of medication history information documented by physicians in case notes of systematic samples of 900 patients that were stratified over 9 Medical outpatients Units at a premier teaching hospital in south western Nigeria. The second phase was an exploratory study involving 10 pharmacists who conducted cross-sectional medication history interview for 324 randomly selected patients.Results49.2% of patients, whose medication history were documented at the baseline, by physicians, were males; while 50.3% of patient interviewed by pharmacists were male. Mean age (SD) of males and females whose medication histories were documented by physicians and pharmacists were 43.2 (SD=18.6), 43.1 (SD=17.9) years and 51.5 (SD=17.6), 52.1 (SD=17.4) years respectively. The frequency of medication history information documented by pharmacists was significantly higher for twelve of the thirteen medication history components (P < 0.0001). These include prescription medicines; over the counter medicines; source of medicines; adverse drug reactions; allergy to drugs, allergy to foods, allergy to chemicals; patient adherence; alcohol use; cigarette smoking; dietary restrictions and herbal medicine use. The depth of medication history information acquired and documented by pharmacist was significantly better for all the thirteen medication history components (P<0.0001).ConclusionPharmacists’ participation resulted in significant increase in frequency and depth of medication history information documented in a developing setting like Nigeria. The new medication history evaluation criteria proved useful in assessing the impact of pharmacists’ participation.