Opium and its many preparations have been used for the treatment of pain for over 2000 years. Many of its pharmacological effects, such as euphoria and sedation, appear to have been known as far back as at least 3000 bc, at the time of the ancient Sumerians and Minoans, and mention is made of its analgesic effect in Egyptian mythology. Morphine remains the standard against which all new analgesics are compared. Although newer opioids may have different properties, in particular with regard to their pharmacokinetics, none is clinically superior in relieving pain. Opioids cause a dose-dependent depression of ventilation that is usually referred to as respiratory depression. However, the term opioid-induced ventilatory impairment may be more appropriate as there are three elements to consider rather than just depression of the respiratory center only. Nausea and vomiting are very common adverse effects of opioids and result from activity in the vomiting center located in the brain stem.