Evolution of a rational use of opioids in chronic pain.

Since the isolation of morphine from opium by Serturner in 1806, morphine, which Osler referred to as "God's own medicine", has remained the analgesic of choice for the treatment of severe pain despite its known addictive potential and the attempts to replace it with synthetic or semi-synthetic medications. Its early use was based on clinical experience alone until the identification of the opioid receptors and endogenous opioids 25 years ago. Advances in pharmacology and pharmacokinetics and the development of precision equipment to quantify the parent drugs and their metabolites in plasma have provided a better understanding of the mechanisms of action as well as adverse effects. Clinicians can now select the drug and route of administration to suit the patient's needs. The objective of care now is for clinical observation to be reinforced by randomized controlled trials and evidence-based medicine, but controlled clinical trials are lacking for the use of opioids in chronic pain.