Disclosed is a long-acting composition for the treatment of an animal in need thereof wherein the composition includes a therapeutically effective amount of a bioactive agent, characterised in that the composition includes a non-aqueous carrier and a solvent system comprising castor oil and at least one cyclic amide (preferably N-Methylpyrrolidone(NMP)). Also disclosed is the method for treating a parasitic infection in an animal using the composition where the biological agent is an anthelmintic especially selected from the group consisting of that macrocyclic lactones avermectin, moxidectin, milbemycin, ivermectin, abamectin, doramectin, epinomectin and selamectin.
