Abstract
Summary and Conclusion
The mouse given M99 or morphine exhibited an acute tolerance to the analgesic effects of both drugs. The acute tolerance was built up rapidly following a single injection of M99 or two daily injections of morphine (24 or 48 hr, respectively). Discontinuation of drug dosage for up to 6 weeks resulted in only a slight decrease in tolerance, which returned to its earlier state after a very few daily doses. M99, based upon 15-min peak analgesic effects in these tests, was approximately 2000 times as potent as morphine.
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