Effects of extremely low frequency magnetic field on the development of tolerance to the analgesic effect of morphine in rats.

This study examined the effects of low frequency magnetic field (MF) on tolerance to analgesic effect of morphine in rats. Rats were made tolerant to morphine by injecting morphine (10 mg/kg, s) once daily for 8 consecutive days. Rats were simultaneously exposed to an MF (50 Hz at 1, 50, and 100 μT for 30 min) before, immediately, or 30 min after injection of morphine, and also exposed to a 0.