Illicit cocaine laboratories in South America have been adding phenyltetrahydroimidazothiazole enantiomers (levamisole and/or tetramisole) to refined illicit cocaine for over 8 years. A chiral capillary gas chromatographic methodology is presented for phenyltetrahydroimidazothiazole enantiomer determination in illicit cocaine samples and in the urine of cocaine abusers. Illicit cocaine samples (N = 752) and urine specimens from cocaine abusers (N = 50) that contained phenyltetrahydroimidazothiazole were analyzed for enantiomeric composition. Legitimate commercial preparations of phenyltetrahydroimidazothiazole are either 100% levamisole or a 50:50 mixture of levamisole and dexamisole (tetramisole). Specimens that contain phenyltetrahydroimidazothiazole mixtures that are other than 50:50 preparations will be enhanced in one isomer over the other, and they are referred to as either "levamisole-enhanced" or "dexamisole-enhanced". Cocaine samples were found to contain levamisole (N = 495, 66%), tetramisole (N = 143, 19%), and levamisole-enhanced enrichment (N = 114, 15%); urine specimens contained levamisole (N = 23, 46%), levamisole-enhanced enrichment (N = 10, 20%), and dexamisole-enhanced enrichment (N = 13, 26%). The toxicological and forensic aspects of these findings are discussed.