Magnesium hydroxide for constipation (Phillips' Milk of Magnesia)

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Magnesium hydroxide is a laxative for occasional use only.

Take a dose at bedtime with a glassful of water.

If you are taking other medicines, do not take magnesium hydroxide within two hours (before or after) of your other medicines.
Type of medicineA laxative
Used forConstipation
Also calledPhillips'® Milk of Magnesia
Available asMixture

Constipation is a common problem. It can mean either going to the toilet less often than usual to empty your bowels, or passing hard or painful stools. Constipation can be caused by a number of things. Not eating enough fibre or not drinking enough fluid can cause constipation. Some conditions (such as pregnancy) can cause constipation, as can a lack of exercise or movement (such as being ill in bed), and some medicines. Often, increasing the amount of fibre in your diet (such as by eating more fruit, vegetables, cereals and wholemeal bread) and drinking plenty of water each day can effectively prevent or relieve constipation. A laxative such as magnesium hydroxide can also be helpful, if used occasionally.

Magnesium hydroxide works by drawing fluid into your bowel, which makes your stools softer and easier to pass. It can be bought without a prescription at pharmacies and other retail outlets.

To make sure that this is the right treatment for you, ask a pharmacist or doctor for advice before you start taking magnesium hydroxide if:

  • You are pregnant. This is because if you are expecting a baby, medicines should only be taken on the advice of a doctor.
  • You have problems with the way your kidneys work, or problems with the way your liver works.
  • You have severe tummy (abdominal) pain, cramping, or sickness.
  • You are taking any prescription medicines. This is because magnesium hydroxide can reduce the absorption of many medicines.
  • You have ever had an allergic reaction to a medicine.
  • Before you start taking the mixture, read the manufacturer's printed information attached to the pack. It will give you more information about magnesium hydroxide and how to take it.
  • Shake the bottle well to make sure the medicine is evenly mixed before pouring out a dose. The dose for an adult is 30-45 ml (six to nine 5 ml spoonfuls). The dose for a child aged 3-12 years is 5-10 ml (one or two 5 ml spoonfuls), although magnesium hydroxide should only be given to a child on the advice of a healthcare professional. Swallow the dose with a glass of water or milk at bedtime. The water/milk helps the magnesium hydroxide to work so it is important that you drink a glassful. It is also especially important if you are elderly, or at risk of becoming lacking in fluid in the body (dehydrated) for any reason.
  • Magnesium hydroxide is a laxative for occasional use only - do not take it for more than three consecutive days. It must not be taken to prevent constipation or to 'keep you regular'. If you are concerned about your bowel habit, please speak with your doctor for advice.
  • Do not take magnesium hydroxide at the same time as any other medicines. This is because it can interfere with the way your body absorbs other medicines and may stop them from working properly. Try to leave at least two hours both before and after a dose of magnesium hydroxide before you take any other medicines.
  • It is important for you to drink plenty while you are constipated. Adults should aim to drink at least two litres (about 8-10 cups) of fluid per day. Most sorts of drink will do but as a start, try just drinking a glass of water 3-4 times a day in addition to what you normally drink.
  • Try to eat a balanced diet containing high-fibre foods such as wholemeal and wholegrain breads and cereals, fruit and vegetables, brown rice and wholemeal pasta. If you are not used to a high-fibre diet, it may be best to increase the amount of fibre you eat gradually.
  • Keeping your body active will help you to keep your digestive system moving, so try to take some regular daily exercise.
  • You may wish to include some foods in your diet that contain sorbitol. Sorbitol is a naturally occurring sugar. It is not digested very well and draws water into your bowel which has an effect of softening stools. Fruits (and their juices) that have a high sorbitol content include apples, apricots, gooseberries, grapes (and raisins), peaches, pears, plums, prunes, raspberries and strawberries.
  • Food such as pastries, puddings, sweets, cheese and cake can make constipation worse and are probably best avoided.
  • You can read more about how to prevent or treat constipation in the separate condition leaflets called Constipation in Adults and Constipation in Children.

Occasionally, people who have taken magnesium hydroxide have experienced digestive symptoms such as mild diarrhoea and stomach cramps. If either of these happens to you, stop taking it. Other side-effects are unlikely but if you do experience any other symptoms, speak with a doctor or pharmacist for further advice.

  • Keep all medicines out of the reach and sight of children.
  • Store in a cool, dry place, away from direct heat and light.
  • Check the label on the bottle for an expiry date. Once a bottle of mixture has been opened, it may need to be used within a period of time, or discarded.

If you suspect that you or someone else might have taken an overdose of this medicine, go to the accident and emergency department of your local hospital. Take the container with you, even if it is empty.

If you are having an operation or any dental treatment, tell the person carrying out the treatment which medicines you are taking.

Do not keep out-of-date or unwanted medicines. Take them to your local pharmacy which will dispose of them for you.

If you have any questions about this medicine ask your pharmacist.

Further reading & references

  • British National Formulary; 69th Edition (Mar 2015) British Medical Association and Royal Pharmaceutical Society of Great Britain, London

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Helen Allen
Current Version:
Peer Reviewer:
Dr John Cox
Document ID:
3817 (v24)
Last Checked:
Next Review:
The Information Standard - certified member

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