The causes of constipation are either primary or secondary.

  • Primary causes include dietary factors, colonic motility (activity of movement), absorption of fluid in the colon, function of the anus and mechanism of evacuation, and behavioural and psychological factors.
  • Secondary causes include some types of medication, an underactive thyroid, low calcium levels, diabetes mellitus, pregnancy, spinal cord injury, or neurological conditions such as Parkinson’s disease and multiple sclerosis.

Diet and Fluid

The relationship of fibre to constipation is actually rather complex. Some constipated people will respond very well to increasing the amount of fibre in their diets but others will not (see Change your diet). Similarly, increasing fluid in your diet does not always help constipation. Avoid dehydration, but bear in mind that increasing fluid intake may just make you urinate more rather than defecate more.

Irritable Bowel Syndrome

Irritable bowel syndrome can be a cause of constipation (known as IBS-C). See our section on irritable bowel syndrome (IBS) for more information.

Evacuation Problems

Problems in the pelvic floor muscles can affect the evacuation of stools from the rectum. This is often termed obstructed defecation syndrome. For more information see rectocoele.

Behavioural and Psychological Factors

Persistently ignoring the need to defecate, what doctors call ‘poor bowel habit’, can exacerbate constipation. You might ignore the urge to open your bowels because you are too busy or because you dislike using a lavatory away from home or near other people (known as ‘shy bowel’). For more information see Toilet training and Shy bowel.
Stress or lifestyle changes can cause constipation. Depression can also bring about constipation. Nerves link the brain to the gut. Reduced activity of these nerves in depression affects the muscle activity of the bowel and results in constipation in some people.

Painful Conditions

Pain from conditions such as an anal fissure can cause constipation.


Many medicines can cause constipation. Common culprits are:

  • pain-killers or cough medicines containing codeine and ibuprofen
  • antacids (for indigestion) containing aluminium or calcium
  • iron tablets
  • some antidepressants (tricyclic and monoamine oxidase inhibitor types) and tranquillizers
  • some drugs for Parkinson’s disease and for epilepsy (Parkinson’s disease can itself cause constipation)
  • some diuretic drugs (for high blood pressure or heart failure)
  • drugs for high blood pressure (verapamil, nifedipine, atenolol, clonidine)
  • antihistamines (e.g. for hayfever)
  • antispasmodics (dicyclomine, mebeverine, peppermint oil)
  • calcium supplements (e.g. for osteoporosis).


About 1 in 3 pregnant women has constipation, probably because of the hormone progesterone. A simple laxative may help if constipation in pregnancy is causing problems, but you should speak to your doctor first.

Stopping Smoking

Almost 1 in 10 quitters experience constipation for a while (Addiction 2003;98:1563–1567).


First published on:
Reviewed and edited by: Dr Kevin Barrett
Last updated: October 2020

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