Opioid effects may be classified as either desirable or undesirable. The undesirable effects, are in the classic sense, termed opioid toxicity or side effects. In the following review, current and new knowledge concerning mechanisms and clinical aspects of the side effects are described and classified. In particular, classification of side effects by their clinical significance in short-term treatment of the opioid-naive patient and long-term treatment of the chronic pain patient is described systematically. Furthermore, new experimental and clinical knowledge of morphine-induced hyperalgesia, allodynia and myoclonus are presented. Finally, the phenomena of tolerance and dependence are discussed.