Because the Government is required by law to specify the exact chemical structure and name of any substance it wishes to control, kitchen chemists are able to develop new drugs with minor modifications that place them beyond the reach of the law. A major class of these drugs, the fentanyl analogs, are simple derivatives of a very potent analgesic-anesthetic introduced in 1968. The illicit fentanyls, such as China white and Mexican Brown, are being sold to addicts on the streets. While distinct from other narcotics, they are equivalent in primary effects, side effects, and toxicity. These analogs are the drug of choice for many heroin users because they are inexpensive, potent, difficult to detect by routine analytical methods, and often quite legal. These and other designer drugs are increasing in use, have been responsible for overdose fatalities, and represent a significant hazard and public health problem.