We evaluated the effects of pharmacist intervention for adverse drug reaction detection and exacerbation avoidance, as well as the severity and outcome of reactions based on analyses of pharmacist involvement in a collaborative approach to medicine. Of 5436 cases with pharmacist involvement, adverse drug reaction prevention was seen in 440, accounting for 8.1%, and exacerbation avoidance in 213, accounting for 3.9%. We concluded that pharmacist involvement contributes to detect adverse drug reactions and avoid exacerbation, and improves pharmacotherapy safety. We also analyzed 131 cases in which the course after intervention was followed. When categorized by adverse drug reaction severity, Grade 1 and 2 were the same at 45.8%, Grade 3 at 8.4%, respectively. Those findings suggested that pharmacist intervention contributes to early detection of an adverse drug reaction. Also, the relationship between clues for detecting adverse drug reactions by a pharmacist and their severity showed that objective evaluations such as clinical laboratory test results, physical assessments and medication history were important for detecting reactions that became more serious. Patients recovered or recovering from an adverse reaction comprised 76.4%, indicating that pharmacist intervention contributed to exacerbation avoidance and improvement. Our findings revealed the effects of pharmacist intervention for adverse drug reaction detection and exacerbation avoidance, and for safety improvement of pharmacotherapy. Additionally, we considered it necessary for the future pharmacist intervention to improve skills of assessing an adverse drug reaction objectively.