The effect of nifedipine on fentanyl-induced analgesia in rats.

We investigated the modulatory effect of subcutaneous administration of different doses of nifedipine (1.5 or 15 mg/kg), a calcium channel blocker, on fentanyl (15 micrograms/kg), an opioid mu-agonist, in rats. The drugs were tested individually as well as in combination. Normal saline and dimethyl sulfoxide were used as controls. Nociceptive sensitivity was assessed by tail-flick technique. No analgesic effect by nifedipine alone at any of the three doses was observed. A slight analgesic effect by fentanyl alone, lasting up to 60 minutes, was noted in the tail-flick responses only (p < 0.05). The combination of both drugs did not enhance or prolong the analgesic effect of fentanyl. The same results were obtained in a supplementary study using higher doses of fentanyl (30 micrograms/kg) with nifedipine (15 mg/kg). Our results suggested that, unlike other calcium channel blockers, nifedipine injected subcutaneously does not enhance fentanyl analgesia.