A drug delivery composition comprises a drug elution rate-controlling excipient comprising an elastomeric polymer defining a reservoir. The reservoir contains at least one discrete solid dosage form comprising at least one active pharmaceutical ingredient (API) and one or more non-polymeric sorption enhancers (e.g., one or more non-polymeric acids, bases, or salts). The drug delivery composition is in an implantable dosage form. A method of treating or preventing one or more diseases or conditions in a subject comprises implanting the drug delivery composition into a subject to systemically deliver a therapeutically effective amount of the API to the subject for a period of time of at least one month at a pseudo-zero order elution rate.
