Endogenous opioid withdrawal in the Jarisch-Herxheimer reaction.

Meptazinol, a partial opioid agonist, diminishes the Jarisch-Herxheimer reaction in relapsing fever whereas naloxone, a pure opioid antagonist, is virtually inert. Because endogenous opioid activity is probably increased in the acute phase, the efficacy of meptazinol is unlikely to be due to agonist activity. However, it is possible that in such a severe reaction there may be an exhaustion of endogenous opioids, giving rise to a naturally occurring quasi-morphine-withdrawal syndrome which responds to meptazinol.