Opium has been known and used as an analgesic since the beginning of times. It consists of the dried milk juice derived from the unripe seed capsules of the opium poppy, Papaver somniferum. Opium contains a number of alkaloids, of which only a few — morphine, codeine, noscapine and papaverine — are of clinical use. In 1803, Serturner isolated a crystalline sample of the main constituent alkaloid, morphine, which was later shown to be almost entirely responsible for the analgesic activity of crude opium. Although several new synthetic strong opioids have occured in the past century, morphine is still the most widely used opioid and remains the “gold standard” when effects of other opioid analgesics are to be compared. Due to a more liberal approach to opioid analgesics especially in cancer patients, but within the later years also in patients with pain of chronic non-malignant origin, the consumption of opioids is vastly increasing. However, there are still countries where access to opioids are extremely limited. As morphine and other opioid drugs today are used for longer periods and in higher doses in patients suffering from chronic pain a number new clinical observations concerning analgesic action and side effects have occurred recently. Furthermore basic science has increased the knowledge of mode of action and toxicity considerably. Despite having been known for so long, morphine is still a puzzling drug to the scientists and clinicians. The scope of this article is to review morphine metabolism and update the clinical implications of morphine and its major metabolites.