Levorphanol

Levorphanol (Levo-Dromoran®) is an opioid medication used to treat severe pain. It is the laevorotary stereoisomer of the synthetic drug morphinan (Dromoran) and a pure opioid agonist, first described in Germany in 1946 as an orally active morphine-like analgesic. Morphinan is the parent drug and prototype of a large series of opioid and/or NMDA pure or mixed agonists used in medicine including nalbuphine, butorphanol, dextromethorphan, and others. One morphinan derivative, cyclorphan was found to be highly hallucinogenic and psychotomimetic and have other untoward effects and of course was never marketed as an analgesic.

Levorphanol has the same properties as morphine with respect to the potential for habituation, tolerance, physical dependence and withdrawal syndrome. 30 mg of oral morphine is roughly equianalgesic to 4 mg of oral levorphanol. The laevo isomer is the source of the narcotic properties of the racaemic drug Dromoran, but the dextro isomers are also useful in medicine: in addition to acting on sigma opioid receptors, the O-methyl derivative of its dextrorotary isomer, dextromethorphan, acts as an NMDA receptor antagonist. Typical doses of levorphanol include 2 mg by mouth or subcutaneous injection every 6 to 8 hours.

Levorphanol has affinity to μ, κ, and δ opioid receptors, but lacks complete cross-tolerance with morphine. It has a mean duration of action from 4-7 hours and for this reason is useful in palliation of chronic pain and similar conditions. Levorphanol has an oral to parenteral effectiveness ratio of 2:1, one of the most favourable of the strong narcotics. Its NMDA actions, similar to those of the phenylheptylamine open-chain narcotics such as methadone and ketobemidone, make levorphanol useful for types of pain that other analgesics may not be as effective against.

It is chemically related to dextromethorphan, an antitussive which is not an analgesic. Dextromethorphan is a salt of the methyl ether dextrorotatory isomer of levorphanol.