Levorphanol tartrate is the levorotatory form of methorphan and is approximately 7.5 times more potent than morphine orally. The loss of the 4,5-epoxide and the 7,8-double bond allows levorphanol greater flexibility and presumable leads to the increased binding affinity at all opioid receptor subtypes compared with morphine. The plasma half-life of lev-orphanol is about 6 to 8 hours but displays great interperson variability and may increase upon repeated dosing. The excretion of levorphanol is dependent on the kidneys, so caution must be used in renally compromised patients.67
The analgesic effect of levorphanol may not match the long plasma half-life, and patients must be closely monitored for drug accumulation and respiratory depression. Levorphanol has strong agonist activity at the ¡¿-, k-, and S-opioid receptors and has also been shown to be a noncompetitive N-methyl-d-aspartate (NMDA)-receptor antagonist.68 The pharmacodynamic properties of levorphanol are sufficiently different to make it an attractive alternate for patients that receive inadequate pain relief from morphine. Levorphanol is available as a 2-mg oral tablet (Levo-Dromoran), and a 2-mg/mL solution for injection.
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