The dosage form of claim 10 wherein the polymer is a poly(ortho ester) composed of the monomer residues of:(a) 3,9-bis-(ethylidene)-2,4,8,10-tetraoxaspiro 5,5! undecane; (b) ivermectin; (c) tetraethylene glycol; (d) 1,6-hexanediol; (e) 1,2,6-hexanetriol. 12.