MMG22 does not produce tolerance to its analgesic effect following prolonged administration. Tumor-bearing mice were treated twice daily with s.c. injections of MMG22 (0.1 mg/kg; n = 8) (left panel) or morphine (5 mg/kg; n = 8) (right panel) for nine consecutive days, and paw withdrawal frequency was determined before and at 60 min after the injection on days 1, 3, 6, and 9. No analgesic tolerance occurred in mice given MMG22. Black bars are paw withdrawal frequencies before injection and grey bars are withdrawal frequencies after injection. * indicates a significant difference from pre-injection values (Student-Newman Keuls test, p < 0.001.