Opioids, defined as drugs that stimulate opioid receptors, are primarily used in the treatment of moderate to severe pain. They induce central nervous system (CNS) adverse effects which can be divided into three groups. The first group includes effects that lower the level of consciousness-sedation, drowsiness and sleep disturbance. The second group affects the thinking process and the ability to react-cognitive impairment, psychomotor impairment, delirium, hallucinations, dreams and nightmares. The third group is of the direct toxic effects of opioids on neurons and includes myoclonus (perhaps), hyperalgesia and tolerance. This review addresses the incidence, possible mechanisms, and treatment of each of these groups of opioid-induced adverse effects.