A method of treatment, such as treating an estrogen-related disorder or a psychotic disorder, by implanting a reservoir-based drug delivery composition into a subject to systemically deliver a therapeutically effective amount of an active pharmaceutical ingredient (such as an aromatase inhibitor or risperidone) to the subject for a long period of time (e.g., one month or one year). The drug delivery composition may include a rate-controlling excipient (e.g., an elastomeric polymer) defining a reservoir containing at least one discrete solid dosage form (e.g., one or more pellets), which includes an active pharmaceutical ingredient and optionally, a sorption enhancer.
