In patients with renal or hepatic failure, the pharmacokinetics of opioids may be affected in several ways, leading to the necessity to correct the dose. The liver is the major site for biotransformation of most opioids. The major metabolic pathway is oxidation. Exceptions to this are morphine and buprenorphine, which undergo primarily glucuronidation, and remifentanil which is cleared by esther hydrolysis. The hydrophilic metabolites are predominantly excreted by the kidneys and may accumulate in patients with renal insufficiency. Some metabolites such as morphine-6-glucuronide (M6G) or normeperidine are active opioid agonists. With high concentrations they may cause narcotic effects or respiratory depression. In addition, special risks are known for normepridine that has been shown to exert neurotoxic effects with the risk of seizures. Few cases of respiratory depression following the administration of codeine, dihydrocodeine and tramdol have been reported. The elimination half-life of these drugs was prolonged. Lastly, the disposition of methadone, buprenorphine, fentanyl, sufentanyl and remifentanil appears to be unaffected in renal failure. In patients with hepatic cirrhosis it has been shown that oxidation of opioids is reduced, resulting in a decreased drug clearance (meperidine, propoxyphene, pentazocine, tramadol and alfentanil) and increased oral bioavailability due to reduced first-pass metabolism (meperidine, propoxyphene, pentazocine, dihydrocodeine). Although glucuronidation is thought to be less affected in liver cirrhosis, the clearance of morphine was found to be decreased and its oral bioavailability increased. The consequence of reduced drug metabolism is the risk of accumulation in the body, especially with repeated administrations. As for patients with renal failure, special risks are known for meperidine with potential accumulation of normeperidine, which can cause seizures, and for propoxyphene for which several cases of hepatotoxicity have been reported. On the other hand, the analgesic activity of codeine and tilidine depends on transformation into the active metabolites, morphine and nortilidine. In the case of reduced metabolism in chronic liver disease, the analgesic action of these drugs may be compromised. Lastly, the disposition of a few opioids, such as fentanyl, sufentanil, and remifentanil, appears to be unaffected in liver disease.