Decrease in analgesic effect of nifedipine following chronic morphine administration.

The analgesic effect of nifedipine, a calcium antagonist, was compared in the acetic acid-induced writhing test with control and morphine-tolerant mice. Nifedipine inhibited the writhing syndrome less effectively in mice made tolerant to morphine. The results support the notion that inhibition of the calcium influx is one of the causes of the analgesic action of morphine and that its chronic administration causes an increase of calcium entry.