**4. Opioid analgesic agents**

In 1680, Sydenham wrote "Among the remedies it has pleased Almighty God to give to man to relive his suffering, none is so universal and so efficacious as opium" [2, 26]. Hundreds of years later, this statement is still valid, and opioids are the cornerstone of pain management. The beneficial effects have been well studied for centuries, as their toxicity and also the potential for abuse.

Opioid analgesics are usually used to relieve moderate to severe pain particularly of visceral origin. Repeated administration may cause dependence and tolerance, but this is not deterrent in the pain control of terminal illness. Regular use of a potent opioid may be appropriate for certain cases of chronic non-malignant pain; treatment should be supervised by medical staff and the patient should be assessed regularly. However, due to concerns about inducing opioid toxicity or addiction and sometimes due to poor understanding of the pharmacology features of these drugs, opioids are often inadequate used in clinical practice [27, 28].
