Repeated low doses of morphine induce a rapid tolerance in arthritic rats but a potentiation of opiate analgesia in normal animals.

The effect of repeated low doses of morphine (0.3-3 mg/kg s.c., twice daily for 4 days) on subsequent sensitivity to the antinociceptive effect of morphine was tested in arthritic and normal rats. Chronic morphine induces tolerance in arthritic rats. This tolerance develops rapidly since it is clearly present after one day of treatment. By contrast, in normal animals similarly treated, a potentiation of morphine analgesia was observed.