Levorphanol

Levorphanol (Levo-Dromoran) is an opioid medication used to treat severe pain. It is the levorotatory stereoisomer of the synthetic morphinan (Dromoran) and a pure opioid agonist, first described in Germany in 1948 as an orally active morphine-like analgesic. Morphinan is the parent drug and prototype of a large series of opioid and/or NMDA antagonists and opioid agonists used in medicine including nalbuphine, butorphanol, dextromethorphan, and others.

Levorphanol labeled with tritium was used in the research which led to the discovery of opioid receptors in the human nervous system, including the first study published in 1971.

Levorphanol has the same properties as morphine with respect to the potential for habituation, tolerance, physical dependence and withdrawal syndrome. It is 4 to 8 times as potent as morphine and has a longer half-life. Approximately 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 racemic drug Dromoran, but the dextro isomers are also useful in medicine: in addition to acting on sigma receptors, the O-methyl derivative of its dextrorotary isomer, dextromethorphan, is a common NMDA receptor antagonist and pro-drug to dextrorphan. Oral doses of levorphanol come in 2 mg and 4 mg strengths 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. The duration of action is generally long compared to other comparable analgesics, and varies from 4 hours to as much as 15 hours. For this reason levorphanol 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; levorphanol's sigma receptor, SNRI properties make it even more useful particularly for neuropathic pain.