Pentazocine is a synthetically prepared prototypical mixed agonist-antagonist narcotic (opioid analgesic) drug of the benzomorphan class of opioids used to treat moderate to moderately severe pain. Pentazocine is sold under several brand names, such as Fortral, Talwin NX (with the μ-antagonist naloxone, will cause withdrawal in opioid dependent persons on injection), Talwin, Talwin PX (without naloxone), Fortwin (Lactate injectable form) and Talacen (with acetaminophen). This compound may exist as one of two enantiomers, named (+)-pentazocine and (-)-pentazocine. (-)-pentazocine is a κ opioid receptor agonist, while (+)-pentazocine is not, instead displaying a ten-fold greater affinity for the σ receptor. Talwin PX is the main pentazocine pharmaceutical in Canada, where laws and regulations prohibit the addition of naloxone to the formulation for non-therapeutic purposes. Related drugs include phenazocine, dezocine, cyclazocine and several chemicals used in research on the central nervous system.