Q. Why do medicines with codeine tend to cause constipation?



A. Codeine is an opiate, a constituent of opium, and like all drugs of its kind it has a complex interaction with the human body that tends to slow down its systems, including the digestive system. In fact, one of its medical uses is treating diarrhea.

A 2002 review in the journal Pharmacotherapy described how opiates and drugs that resemble them, called opioids, are thought to cause constipation.

One important factor in constipation is a reduction in the speed of peristalsis, the ripples of muscle contractions and relaxations that move food along the digestive tract. Opioid receptors on the smooth muscles of the gut are believed to slow down both contraction and relaxation. Naloxone, a drug that specifically counters the effects of opioids on these receptors, has been used to treat constipation of unknown origin.

It has also been suggested that opioids interfere with the normal supply of water and electrolytes in the lining of the intestines. Another possible factor is opioid interference with receptors in the anal canal that normally detect an accumulation of waste and signal the internal anal sphincter valve to relax and discharge it.