Heroin hydrochloride is approximately twice as potent as morphine sulfate, and acts slightly faster but for a shorter duration than morphine. Although patients with chronic pain due to advanced cancer differ from cancer patients with postoperative pain in terms of their degree of tolerance to the analgesic effects of morphine and heroin and their reports of various elements of mood, there is, thus far, no indication that heroin has any unique advantage over morphine in terms of side effect occurrence or effects on mood at equianalgesic doses. Both drugs improve mood provided they are administered in doses which result in analgesia. While there appears to be some slight difference in the spectrum of side effects observed after heroin as compared to morphine, heroin and morphine share the most common side effects. The incidence of side effects following both drugs appear to be highest among those effects which are primarily somatic and undesirable. The use of visual analog scales concurrent with categorical pain and pain relief scores provides a means for the finer estimation of relative analgesic potency and time action. The results of these studies are in general agreement with those of other investigators. Where apparent differences exist they can usually be explained on the bases of differences in methods and subject populations.