Opioid analgesics are strong pain-relieving medications used to treat moderate to severe acute and chronic pain. Drugs in this class include morphine, fentanyl, oxycodone, codeine, hydromorphone and heroin. They tend to be effective in most people, however, their use is limited by side effects and the potential for abuse and addiction.
Central Nervous System Side Effects
Side effects in the central nervous system, which includes the brain and spinal cord, accompany the desired effects on pain. Opioids cause drowsiness, although at times this may be a sought out effect rather than a side effect. They may have a dissociative effect or cause mental clouding in which the environment is perceived as less real. In addition, these drugs produce respiratory depression with large doses leading to cessation of breathing. They also bind to receptors in the brain that cause nausea and vomiting. Meperidine can cause seizures.
Gastrointestinal Tract Side Effects
In the condition called ileus, the motility, or movement, of the GI tract is decreased by opiates. This causes significant constipation that can lead a person to discontinue or largely taper down the use of the drugs in spite of significant pain. According to an article in "The New England Journal of Medicine" by Berde and Nurko, these side effects may respond to oral doses of the drug methylnaltrexone.
Cardiovascular Side Effects
Certain opiates cause cells of the immune system to release histamine, which in addition to causing itching, dilates the blood vessels and can drop the blood pressure, leading to lightheadedness, particularly when going from lying down to sitting to standing, a condition called orthostatic hypotension. Occasionally, high doses can slow the heart beat down.
Pulmonary Side Effects
Typical doses for pain do not have direct effects on the lungs or chest wall themselves, though as noted above, effects on the breath depress respiration. High doses can cause constriction of the small tubes in the lungs, making breathing more difficult. In addition, rigidity of the muscles of the chest wall, called "stiff chest syndrome," can make breathing, or delivering artificial ventilation, difficult.
Genitourinary Tract and Reproductive Side Effects
In the urinary tract, opioids decrease the amount of urine that is produced and decrease the ability to urinate; in other words, they can cause urinary retention. In addition, their effects on the uterus can prolong the duration of labor.
References
- "Textbook of Anaesthesia"; Aitkenhead, Smith, Rowbotham; 5th Ed. 2007
- "New England Journal of Medicine"; Opioid Side Effects — Mechanism-Based Therapy; Berde and Nurko; May 2008
- The Merck Manuals: Treatment: Pain
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