Phenazocine

Phenazocine (Prinadol, Narphen) is an opioid analgesic, invented in the 1950s, which is related to pentazocine and has a similar profile of effects that include analgesic action and euphoria, but may produce dysphoria and hallucinations at high doses, most likely due to action at κ-opioid and σ receptors. However phenazocine appears to be a much stronger analgesic and with less side effects than pentazocine, probably reflecting a more favourable μ/κ binding ratio. Phenazocine used to be widely used in medicine, mainly supplied as 5mg tablets of the hydrobromide salt, but was discontinued in 2001 and is now no longer used.

Phenazocine is much more potent as an analgesic than pentazocine and other drugs in the benzomorphan series, most probably due to the presence of an N-phenethyl substitution, which is known to boost μ-opioid activity in many classes of opioid analgesics. Consequently phenazocine is some 4x the potency of morphine as an analgesic. Also it does not cause spasm of the sphincter of Oddi, making it more suitable than morphine for the treatment of biliary or pancreatic pain.