What is the mechanism of action of an osmotic laxative?

Laxatives are a type of medicine that can treat constipation.

They're often used if lifestyle changes, such as increasing the amount of fibre in your diet, drinking plenty of fluid and taking regular exercise, have not helped.

Laxatives are available to buy from pharmacies and supermarkets. They're also available on prescription from a doctor.

There are 4 main types of laxatives.

Bulk-forming laxatives

Bulk-forming laxatives work by increasing the "bulk" or weight of poo, which in turn stimulates your bowel.

They take 2 or 3 days to work.

Bulk-forming laxatives include:

  • Fybogel (ispaghula husk)
  • methylcellulose

Osmotic laxatives 

Osmotic laxatives draw water from the rest of the body into your bowel to soften poo and make it easier to pass.

They take 2 or 3 days to work.

They include:

  • lactulose (also called by the brand names Duphalac and Lactugal)
  • macrogol (also called by the brand names Movicol, Laxido, CosmoCol, Molaxole and Molative)
  • polyethylene glycol

Stimulant laxatives

These stimulate the muscles that line your gut, helping them to move poo along to your back passage.

They take 6 to 12 hours to work.

They include:

  • bisacodyl (also called by the brand name Dulcolax)
  • senna (also called by the brand name Senokot)
  • sodium picosulfate

Poo-softener laxatives

This type of laxative works by letting water into poo to soften it and make it easier to pass.

They include:

It's difficult to know whether a particular laxative will work better than another. It depends on the person.

Unless there's a reason why a type of laxative may be more suitable for you than another:

  • start with a bulk-forming laxative
  • if your poo remains hard, try using an osmotic laxative in addition to, or instead of, a bulk-forming laxative
  • if your poo is soft but is still difficult to pass, try taking a stimulant laxative in addition to a bulk-forming laxative

Speak to a GP or pharmacist if you're unsure which laxative to use.

Also see a GP if you're still constipated after trying all of the different types of laxative, or you think your child might benefit from taking laxatives.

Laxatives are not suitable for everyone.

They're not usually recommended for:

Before using a laxative, read about it in our Medicines guide or the patient information leaflet that comes with the medicine to make sure it's safe for you to take.

Find out more about the considerations about laxatives

How you take a laxative depends on the form it comes in.

They're commonly available as:

  • tablets or capsules you swallow
  • sachets of powder you mix with water and then drink
  • a capsule you place inside your bottom (rectum), where it'll dissolve (suppositories)
  • liquids or gels that you place directly into your bottom

Some laxatives have to be taken at certain times of the day, such as first thing in the morning or last thing at night.

Ask a pharmacist for advice if you're not sure how to take your laxative.

If you're taking bulk-forming or osmotic laxatives, it's important to drink plenty of fluids. This is because these laxatives can cause dehydration.

Never take more than the recommended dose of laxatives as this can be harmful and cause side effects.

Ideally, only take laxatives occasionally and for up to a week at a time.

Stop taking a laxative when your constipation improves.

If your constipation has not improved after taking laxatives for a week, speak to a GP.

After taking a laxative, you can make certain lifestyle changes to help stop getting constipated again, such as:

  • drinking plenty of water
  • exercising regularly
  • including more fibre in your diet

These are better ways of preventing constipation than using laxatives.

Do not take laxatives every day to ease your constipation as this can be harmful.

Speak to a GP if you're still constipated after making lifestyle changes.

In some cases, you may be prescribed a laxative to use regularly, but this should always be supervised by a GP or gastroenterologist (a specialist in gut problems).

Like most medicines, laxatives can cause side effects. They're usually mild and should pass once you stop taking the laxative.

The side effects you may get will depend on the type of laxative you're taking, but common side effects of most laxatives include:

  • bloating
  • farting
  • tummy cramps
  • feeling sick
  • dehydration, which can make you feel lightheaded, have headaches and have pee that's a darker colour than normal

Ask a GP for advice if you get any particularly troublesome or persistent side effects while taking laxatives.

Using laxatives too often or for too long can also cause diarrhoea, the bowel becoming blocked by large, dry poo (intestinal obstruction), and unbalanced salts and minerals in your body.

It's often possible to improve constipation without using laxatives.

It may help to:

  • increase your daily intake of fibre – try to eat about 30g of fibre a day; read about how to increase the fibre in your diet
  • add bulking agents, such as wheat bran, to your diet – these will help make your poo softer and easier to pass, although bran and fibre can sometimes make bloating worse
  • drink plenty of water
  • exercise regularly

Find out more about preventing constipation

Page last reviewed: 11 October 2022
Next review due: 11 October 2025

Osmotic laxatives are medications used to treat or prevent constipation. They draw extra water into your stool, making it softer and easier to pass. Osmotic laxatives are available over the counter and by prescription. Examples include Milk of Magnesia, lactulose, and polyethylene glycol (PEG).

This article explains how osmotic laxatives work, how they differ from other types of laxatives, plus the possible risks and side effects.

Illustration by Jessica Olah, Verywell

Constipation occurs when stools are infrequent and hard to pass. The stools will usually be hard and dry. Osmotic laxatives can help relieve constipation by increasing the amount of fluid in the intestines. This, in turn, softens stools and makes them easier to pass.

The term "osmotic" refers to the movement of a fluid through a membrane so that the concentration is equal on both sides. This is how osmotic laxatives work.

In people with constipation, the concentration of water in the wall of the colon and the inside of the colon (called the lumen) will be balanced but too low to compensate for hard, dry stools. This is especially true of people who are don't consume enough water.

Osmotic laxatives alter the balance with substances—such as salts, sugars, and other organic compounds—that encourage the movement of water into the lumen.

In addition to treating constipation, osmotic laxatives are sometimes used for bowel prep (to cleanse the bowel of stool) prior to undergoing colonoscopy.

Osmotic laxatives work by drawing water from the wall of the colon to the inside of the colon. This helps soften stools and makes them easier to pass.

Osmotic laxatives work differently than other types of laxatives in that they are sometimes be used to prevent or treat chronic constipation. The others are generally used for the treatment of occasional constipation.

Other types of laxatives include:

  • Emollient laxatives: These are a type of laxative made with a surfactant called docusate. Surfactants are substances that encourage the spread of fats and water. Docusate increases the passage of water and fats into stools to make them softer.
  • Lubricant laxatives: These are made with oily substances, like mineral oil, that make it easier for stool to slip through the intestine.
  • Stimulant laxatives: These are a type of laxative that relieves constipation by causing the intestines to contract and push out stools.

Osmotic laxatives work differently than emollient laxatives (that draw water and fat to stools), lubricant laxatives (that lubricate stools), and stimulant laxatives (that speed intestinal contractions).

There are several common osmotic laxatives you can use if you have constipation. Each is made with different active ingredients:

  • Polyethylene glycol (PEG): This is an organic compound derived from petroleum that can be safely ingested to manage constipation. Available over the counter, PEG-containing laxatives include Miralax and GlycoLax.
  • Lactulose: This is a type of sugar that is not absorbed by the intestine. Instead, the sugar sits and ferments in the intestines, producing fatty acids that draw water into the lumen. Available by prescription, lactulose-containing laxatives include Cephulac, Duphalac, Kristalose, and many others.
  • Sorbitol: This is another non-absorbable sugar with an action similar to lactulose. Over-the-counter (OTC) and prescription versions are available, including Arlex and GeriCare.
  • Magnesium citrate: Magnesium citrate is magnesium in salt form combined with citric acid. The salts help draw water into the lumen. OTC versions include Citrate of Magnesia, Citroma, and LiquiPrep.
  • Magnesium hydroxide: This is a milder form of magnesium sold under the brand name Milk of Magnesia. Available over the counter, Milk of Magnesia is also used as an antacid.

There are several different active ingredients used in osmotic laxatives, including polyethylene glycol (Miralax), lactulose (Cephulac), Sorbitol (Arlex), magnesium citrate (Citrate of Magnesia), and magnesium hydroxide (Milk of Magnesia).

As with all drugs, osmotic laxatives can cause side effects. Most are relatively mild and will resolve on their own within a couple of days.

Common side effects of osmotic laxatives include:

  • Nausea
  • Bloating
  • Abdominal cramping
  • Flatulence
  • Diarrhea

The overuse of osmotic laxatives can cause dehydration and the loss of electrolytes like sodium, calcium, and potassium. These are some of the minerals that the body needs to regulate heartbeats, muscle contractions, and other key functions.

Although not approved for such, osmotic laxatives like Miralax are sometimes used for the long-term management of chronic constipation. The other osmotic laxatives are generally intended for short-term use.

Speak with your healthcare provider to ensure that you are using any laxative correctly, whether it is over-the-counter or prescription.

Common side effects of osmotic laxatives include nausea, bloating, cramping, flatulence, and diarrhea. The overuse of osmotic laxatives can lead to dehydration and other complications.

Osmotic laxatives can help treat or prevent constipation by drawing water into the colon. This action helps soften stools and makes them easier to pass. Some osmotic laxatives can be used for bowel preparation to help clear the colon of stool in advance of a colonoscopy.

There are different types of osmotic laxatives that contain different active ingredients. These include polyethylene glycol (PEG), lactulose, sorbitol, magnesium citrate, and magnesium hydroxides. Some (like Miralax and Milk of Magnesia) are available over the counter, while others (like Cephulac and Kristalose) are available by prescription only.

Osmotic laxatives can cause side effects like nausea, bloating, cramping, gas, and diarrhea. The overuse of osmotic laxatives can cause dehydration and other potentially serious complications.

If you are thinking about using osmotic laxatives, be sure to follow the dosing instructions carefully. To avoid complications, use a laxative only when needed.

If you have chronic constipation, see your healthcare provider so that they can identify the underlying cause. In some cases, the condition can be improved with diet, exercise, and increased fluid intake. Others may require treatments that extend beyond the use of laxatives.