Crimes: drugs; illicit use of xylazine; prohibit, and provide penalties. Amends secs. 7103, 7105, 7106, 7401, 7403, 7403a, 7451, 7453 & 7455 of 1978 PA 368 (MCL 333.7103 et seq.).

HB 4166 prohibits the non-medical use of xylazine (a sedative sometimes mixed with drugs like fentanyl) in Michigan. It defines "illicit use" as any xylazine use not covered under "licit use," which includes legal veterinary applications, pharmaceutical manufacturing, or approved medical treatments. The bill amends Michigan's Public Health Code to make possessing or using xylazine for non-licensed purposes a felony, with penalties ranging from up to 20 years in prison or fines up to $1 million depending on the amount. This directly affects individuals using xylazine outside approved medical or veterinary contexts, aligning penalties with those for other controlled substances like opioids.