Drugs used to relieve constipation or clear the intestine before a medical procedure

Common drugs

    Bulk-forming agents

  • Bran

  • Ispaghula

  • Methylcellulose

  • Sterculia

    Osmotic laxatives

  • Lactulose

  • Magnesium salts

  • Phosphates

  • Sodium citrate

    Faecal softeners/lubricants

  • Arachis oil

  • Liquid paraffin

    Stimulant laxatives

  • Bisacodyl

  • Dantron

  • Docusate sodium

  • Glycerol

  • Senna

  • Sodium picosulfate

Laxatives make faeces pass more easily through the intestines. They are most commonly used to treat constipation, the difficult, infrequent passing of stools that are hard and dry. However, laxatives may be prescribed for other reasons. For example, they may be given to clear the intestine before a colonoscopy, in which an instrument for viewing the colon is passed through the anus. Laxatives may also be prescribed to counteract the constipating effect of other drugs such as morphine or codeine.

Laxatives can be bought over the counter. If you are taking laxatives for constipation, use them only until your bowel movements have returned to normal. If the constipation continues for more than a few days, you should see your doctor. Do not take more than the recommended dose because some laxatives can cause severe abdominal pain. Never give laxatives to children without first consulting your doctor.

What are the types?

Laxatives can be classified into different types, depending on how they work. Bulk-forming agents, osmotic laxatives, and faecal softeners all make stools softer and easier to pass. Stimulant laxatives make the intestinal muscles move faeces more rapidly. Most laxatives are taken orally, but some osmotic and stimulant laxatives may be administered as enemas or suppositories.

Bulk-forming agents

These preparations cause the faeces to retain water, keeping them soft and increasing their volume, and thereby stimulating intestinal muscle action. They are mainly available as granules or powders that are taken orally. It may take several days for these agents to have their full effect.

Bulk-forming agents are mostly prescribed to treat long-term constipation. For example, they may be used in the treatment of irritable bowel syndrome or diverticular disease. You may also be given bulk-forming agents to make passing stools easier after childbirth or abdominal surgery.

Bulk-forming laxatives are the safest type of preparation for the long-term treatment of constipation because their action is similar to the natural action of fibre in food. You should drink plenty of water when taking these laxatives because the bulky stools may otherwise eventually block the intestine. Side effects of bulk-forming laxatives may include excess intestinal gas and abdominal pain and bloating.

Osmotic laxatives

These drugs work by preventing the body from removing water from faeces. As a result, the faeces stay soft and increase in bulk. Osmotic laxatives are available on prescription and over the counter. The most commonly prescribed osmotic laxative is lactulose, a synthetic form of sugar that the body does not absorb. Lactulose can cause side effects such as intestinal gas and abdominal cramps, which may gradually lessen with continued use. In elderly people, long-term use of lactulose can eventually cause dehydration and lead to a chemical imbalance in the blood.

Other drugs that retain water in the intestines, including magnesium salts, phosphates, and sodium citrate, may be used to achieve rapid bowel evacuation, particularly before procedures such as colonoscopy, radiological investigation, or surgery on the lower digestive tract. Some of these laxatives are taken orally, others are given by enema. They work by drawing water into the gut from the body and therefore may cause dehydration. You should be sure to drink plenty of water when taking these laxatives. Side effects may include intestinal gas and abdominal bloating.

Faecal softeners

These laxatives act by softening faeces. They also lubricate faecal matter, enabling it to pass more easily through the intestine. Arachis oil is available over the counter and is administered as an enema. Because the oil is derived from peanuts, you should not use it if you have a nut allergy. Liquid paraffin is also available over the counter and is taken orally. You should not take this laxative over a long period because it can cause anal irritation. Liquid paraffin can also prevent your body from absorbing certain vitamins, which may lead to nutritional deficiencies in the long term.

Stimulant laxatives

These laxatives stimulate the intestinal muscles to contract more strongly, resulting in more frequent bowel movements. Stimulant laxatives are sometimes used to clear the intestines quickly if other drugs have failed to work. Some of these drugs can be bought over the counter. You should not take stimulant laxatives regularly because your body may come to depend on them to stimulate bowel movements. Side effects commonly include abdominal cramps and diarrhoea. Dantron may colour the urine red.


When taking a bulk-forming laxative such as bran, be sure to drink plenty of water; otherwise, the bulky faeces that are produced may cause an intestinal blockage.

Drug Action: How Bulk-forming Laxatives Work



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