Laxatives are a group of medicines that are used to treat constipation. They can be taken by mouth as liquids, tablets, or capsules, or they can be given via the rectum (back passage) - for example, suppositories, or enemas. Suppositories are pellet-shaped laxatives that are inserted into the rectum, via the anus. An enema is a liquid that is inserted into the rectum and lower colon, via the anus.
There are four main groups of laxatives that work in different ways.

  • Bulk-forming laxatives (also known as fibre supplements). For example, ispaghula husk, methylcellulose, sterculia. Unprocessed bran is a cheap fibre supplement.
  • Osmotic laxatives. For example, lactulose, macrogols, phosphate enemas, and sodium citrate enemas.
  • Stimulant laxatives. For example, bisacodyl, dantron, docusate sodium, glycerol, senna and sodium picosulfate.
  • Faecal softeners. For example, docusate sodium, arachis (peanut) oil enemas, and liquid paraffin.

What is constipation?
Constipation is when faeces (stools or motions) become hard, and difficult or painful to pass. Sometimes crampy pains occur in the lower part of your abdomen. You may also feel bloated and feel sick if you have severe constipation.
How do laxatives work?
Bulk-forming laxatives are sometimes called fibre supplements. They increase the bulk of your faeces in a similar way to fibre. They partly work by absorbing water (a bit like blotting paper). The increase in the bulk of your faeces stimulates the muscles in your gut to squeeze faeces along and out of the body.
Osmotic laxatives work by retaining fluid in the large bowel by osmosis (so less fluid is absorbed into the bloodstream from the large bowel).
Stimulant laxatives stimulate the nerves in the large bowel. This then causes the muscle in the wall of the large bowel to squeeze harder than usual.
Faecal softeners work by wetting and softening the faeces.

Which laxatives are usually prescribed or recommended?

As a general rule:

Treatment with a bulk-forming laxative is usually tried first.
If faeces remain hard despite using a bulk-forming laxative, then an osmotic laxative tends to be tried, or used in addition to a bulk-forming laxative.
If faeces are soft but you still find them difficult to pass then a stimulant laxative may be added in.

How long do laxatives take to work?

Bulk-forming laxatives can have some effect within 12-24 hours but their full effect may take several days to develop.

Osmotic laxatives such as lactulose can take up to two days to have any effect so they are not suitable for the rapid relief of constipation.

Stimulant laxatives usually work within 8-12 hours. A bedtime dose is recommended so you are likely to feel the urge to go to the toilet sometime the following morning.

Faecal softeners usually work within 24 to 48 hours.

Laxatives that are given via the rectum (suppositories or enemas) usually work within 15 to 30 minutes. Stronger osmotic laxatives (such as magnesium salts and phosphate enemas) can be used to clear the bowel quickly and in situations such as before bowel surgery.

How long should I take a laxative for?

This depends on what type of constipation you have. Most people only need to take a laxative for a short time, to get over a bout of constipation. Once the constipation eases, you can normally stop the laxative. Some people get into the habit of taking a laxative each day "to keep the bowels regular" or to prevent constipation. This is not advised, especially for laxatives which are not bulk-forming.

What are the side-effects?

Laxatives very rarely cause serious side-effects. Common side-effects include flatulence, cramps, diarrhoea, nausea, and bloating. Most of the side-effects can be avoided or reduced by starting off on a low dose and increasing the dose of oral laxatives gradually.

When taking a laxative

  • Drinking plenty of fluid.
  • Avoiding taking too much.

Who cannot take laxatives?

You cannot take laxatives if you:

Have a blockage in your gut.
Have Crohn's disease or ulcerative colitis, unless specifically advised by your doctor.

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