According to the World Health Organization (WHO) guidelines for patients with moderate or severe pain, morphine has been used as a "gold standard" treatment for cancer pain. Recent clinical experiences have demonstrated that when morphine is used to control pain in cancer patients, psychological dependence is not a major concern. However, undue anxiety about psychological dependence on morphine in cancer patients has caused physicians and patients to use inadequate doses of opioids. In basic research, we reported that the morphine-induced rewarding effects can be dramatically suppressed under a neuropathic pain-like state induced by sciatic nerve ligation and an inflammatory pain-like state produced by intraplantar injection of formalin or carrageenan in rodents. The use of morphine for the treatment of pain is sometimes accompanied with side effects such as emesis, constipation and drowsiness. We show that the lower doses of morphine produce emesis, whereas antinociceptive doses of morphine show no emetic responses in ferrets. These findings provide further evidence that an adequate dose of morphine is useful and safe in a clinical setting.