A synthetic opioid antagonist which is very similar to morphine in chemical structure and is given by injection (usually in the form of its hydrochloride) as an analgesic; (−)-(5 R,6 S,14 S)-9a-cyclobutylmethyl-4,5-epoxymorphinan-3,6,14-triol, C₂₁ H₂₇ NO₄.