Levallorphan

Levallorphan (Lorfan, Naloxiphan) is a drug of the morphinan family which is used as an opioid antidote or antagonist. It acts as an antagonist on μ-opioid receptor and agonist on kappa receptors [hence called [partial agonist]], (Reference : p 602, Goodman & Gilman's The Pharmacological Basis of Therapeutics, ed. 10 from McGraw Hill Medical Publishing Division)and as a result, blocks the effects of stronger agents with greater intrinsic activity such as morphine or endogenous β-endorphin.

Levallorphan was formerly widely used in general anaesthesia, mainly to reverse the respiratory depression produced by the opioid analgesics and barbiturates which are used for induction of surgical anaesthesia, although it is now less common as the newer drug naloxone tends to be used instead.

It was also used in combination with opioids to reduce their side effects, mainly in obstetrics, and when a very small dose of levallorphan is used alongside an opioid full agonist this can in fact produce greater analgesia than when the agonist is used by itself. The combination of levallorphan with pethidine (meperidine) was indeed used so frequently that a standardized formulation was made available, known as Pethilorfan.