Use the MeSH database to find the pharmacological action, ex. hypoglycemic agents, that is tagged with [Pharmacological Action]. Combine this search with your desired term, ex hypoglycemic agents[Pharmacological Action] AND weight gain[MeSH].
Use the MeSH database to find the type of pharmacological action desired. Search this term as a MeSH term and add the name of the drug. Ex. Platelet Aggregation Inhibitors[MeSH] AND aspirin.