The present invention accomplishes this by having multiple molecules of parent drugs attached to carrier moieties and by extending the period during which the parent drug is released and absorbed after administration to the patient and providing a longer duration of action per dose than the parent drug itself. Prodrug conjugates are suitable for sustained delivery of heteroaryl, lactam-amide-, imide-, sulfonamide-, carbamate-, urea-, benzamide-, acylaniline-, cyclic amide- and tertiary amine-containing parent drugs that are substituted at the amide nitrogen or oxygen atom with labile aldehyde-linked prodrug moieties. The carrier groups of the prodrugs can be hydrophobic to reduce the polarity and solubility of the parent drug under physiological conditions.