Opioids are those chemical compounds that exert their mechanism of action through agonist action (a chemical substance that causes a pharmacological response) at the CNS opioid receptors by modulating the ascending (to the brain) and descending (away from the brain) pathways of the spine. The opium poppy was cultivated in 3400 BC. Narcotic is a Greek term for stupor used to describe sleep medications, opioids, and also as a legal term for abused drugs. Opioids are functionally classified as agonists, mixed agonists-antagonists, or antagonists and by their action at the sites.
The receptors are designated as;
Mu-found in the brainstem and affect spinal analgesia, cause sedation, depress respiration, cause euphoria and sedation. Mu1-analgesia, euphoria, serenity and
Mu2-repiratory depression and itching, sedation, anorexia.
Kappa-also mediate spinal analgesia, cause sedation, dypsnea and dependence
Delta- effects are not well studied
Sigma-cause dysphoria and stress induced depression.
Examples of opioid agonists include morphine, codeine, fentanyl, meperidine, and methadone. Naloxone, an antagonist, rapidly reverses opioids. Naltrexone another antagonist is used orally to detoxify opioid addicts. The agonist-antagonist drugs include buprenorphine and nalorphine, pentazocine, nalbuphine and butorphanol and have a high ‘ceiling effect and so a reduced potential for abuse.
They are very useful in the relief of moderate to severe pain but should be used cautiously in chronic pain, often used in low doses with combinations of other analgesics, especially in those patients with long term use of NSAIDs. When prescribed and monitored closely, their use to “relieve pain” rarely causes abuse or addiction The understanding of these terms will be reviewed because they are frequency misinterpreted and misused.
Physical dependence is the characteristic response when long term therapy is suddenly stopped or significantly reduced. It is not synonymous with addiction and can occur without psychological dependence. Addiction is a ‘craving”. It encompasses three behavior patterns; compulsive overuse(loss of control), refusing to comply with other therapies , as if it were the only beneficial component of the therapy (preoccupation with the drug), and continued use despite reactions or consequences(over sedation, constipation, etc.) It is a pattern of behavior not a specific action of the patient.
Tolerance is the necessity to increase drug dosage to achieve the same effect. It is uncommon with opiates in long term treatment, with 60-80%of the patients remaining at stable established doses. If there is a need to increase dosage, it is more likely due to an increase in disease severity and in pain intensity.
The opioids most often used for nonmalignant ,moderate pain usually are in combination with apap(acetaminophen), asa(aspirin) Propoxyphene(Darvon), once commonly prescribed has now been withdrawn due to poor benefit to risk (cardiac, liver) profile. Also of limited use is Pentazocine (Talwin) due to a higher potential of agitation and delirium, and dysphoria, than other choices.
Nonmalignant chronic pain is not due to terminal disease which lasts longer than expected, continuing after healing of an injury or problem. Arthritis, back pain, diabetic neuropathy, headaches, and post-hepatic neuralgia are such examples. Despite healing, pain will continue with a hyper excited nervous system. Opioids block chemical transmitters to the brain and reduce pain severity. Since the response to opioids is patient selective, there is no one best choice, but must be patient individualized.
Common side effects include sedation, constipation, nausea, vomiting and depressed respiration, urinary retention, sexual dysfunction, bradycardia(slowing heart rate), skeletal muscle rigidity, itching-due to direct histamine effect. The tricyclic antidepressants (amitriptyline, nortriptyline) increase blood levels of morphine while erythromycin increases the effects of opioids.
“Pain Management”, Journal of the Florida Medical Association, September 2006, Vol 90 No.2
MPR, Monthly Prescribing Reference, 2011 edition, Pain Management Pocket Reference
The Rx Consultant, “Chronic Nonmalignant Pain, Use of Opioids” May 2002
The Rx Consultant, “Pain Management-Treating Mild-Moderate Pain in the Community”, May 2000
See our other pain medication discussions.