The change in illicit drug use from 1975 to 1978 was studied in Chicago veterans. Barbiturates and amphetamine use has decreased significantly. Nearly a sevenfold increase in pentazocine use has occurred. All but one of the pentazocine users added tripelennamine to the intravenous or subcutaneous injections. Sixty-four instances of medical or psychiatric complications including seizures, abscesses, depression, and psychosis occurred in the 69 pentazocine and tripelennamine users. A possible pharmacologic explanation of the seizures and psychiatric complications is advanced. Most users of pentazocine and tripelennamine started the use in an attempt to withdraw from heroin or because of the poor quality of heroin available.