An osmotic laxative, also used for bowel preparation
Magnesium citrate
An osmotic laxative used for constipation and to clear the bowel before procedures, working by drawing water into the gut.
What is Magnesium citrate?
Magnesium citrate is an osmotic laxative used to relieve constipation and, in stronger use, to empty the bowel before procedures such as a colonoscopy. It works by drawing water into the bowel, which softens the stool and stimulates a bowel movement. The most important things to know are that it causes fluid and salt shifts in the body, so it is used with caution in people with kidney problems and in older people, and you must drink plenty of fluids while taking it. It reliably produces diarrhoea, especially at the doses used for bowel preparation.
Education and reference only. This is a plain-language guide to Magnesium citrate — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Magnesium citrate is a salt-based, osmotic laxative. It is used in two main ways: at lower strength to relieve constipation, and at higher strength to clear out the bowel before procedures such as a colonoscopy or surgery, so the doctor can see clearly. It is taken by mouth, usually as a liquid or made up from a powder. Because it works by pulling water into the bowel and because the body absorbs some of the magnesium, it has effects on the body's fluid and salt balance that mean it needs to be used with some care, particularly in certain people.
How it works
Magnesium citrate is an osmotic laxative, which means it works by drawing water into the bowel. The magnesium salt stays mostly within the gut and pulls fluid in alongside it, which softens and bulks up the stool and stretches the bowel, triggering a bowel movement. At the higher amounts used to prepare for procedures, this produces a strong, watery clear-out. Because water is being shifted into the bowel and some magnesium is absorbed, the medicine affects the body's fluid and salt balance — which is the reason you need to drink plenty and why caution is needed in some people.
Company & origin
Originated / developed by: Various manufacturers.
A salt-based laxative used to relieve constipation and to empty the bowel before procedures such as colonoscopy.
Practical use
How to take Magnesium citrate
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it by mouth as directed, drinking plenty of clear fluids alongside it to stay hydrated.
- For bowel preparation, follow the timing and fluid instructions for your procedure exactly.
- Expect diarrhoea, particularly with the larger amounts used before procedures, and stay near a toilet.
- Tell your prescriber if you have kidney problems or are elderly, as it is used with extra caution.
- Use it for short-term or occasional relief of constipation rather than every day, unless advised.
Weighing it up
Advantages & disadvantages of Magnesium citrate
Advantages
- Reliably relieves constipation and effectively empties the bowel before procedures.
- Works fairly quickly by drawing water into the bowel.
- Taken by mouth as a liquid or made-up drink.
Disadvantages
- Causes fluid and salt shifts, so plenty of fluids must be taken alongside it.
- Used with caution in people with kidney problems and in the elderly, who can build up too much magnesium.
- Produces strong, watery diarrhoea, especially at bowel-preparation strength.
Practical use
Good to know
The dominant point with magnesium citrate is that it causes shifts in the body's fluids and salts, so two things matter most: drinking plenty of clear fluids while taking it, to avoid becoming dehydrated, and using it with caution in people whose kidneys do not work well or who are elderly, because they are more likely to build up too much magnesium or become dehydrated. Used for constipation it acts fairly quickly, and used for bowel preparation it produces a strong, watery diarrhoea, so being near a toilet is sensible. It is meant for short-term or occasional use rather than long-term daily use for constipation. Always follow the timing and fluid instructions carefully, especially before a procedure.
Who should not take it / use with caution
- People with serious kidney problems should not use it, or only under close medical supervision, because magnesium can build up.
- People with a known or suspected blockage in the bowel should not use it.
- It is used with caution in the elderly, the frail and people prone to dehydration.
Monitoring
- Making sure enough fluid is taken to avoid dehydration.
- Taking extra care, and checking salts and kidney function where needed, in the elderly or those with kidney problems.
- Reviewing ongoing use, as it is meant for short-term rather than long-term daily use.
Side effects
- Diarrhoea, which is expected, along with cramping tummy pain and wind.
- Dehydration if not enough fluid is taken alongside it.
- Disturbances in the body's salts, including a build-up of magnesium, especially with poor kidney function.
Key interactions
- It can reduce the absorption of some other medicines, such as certain antibiotics, so separate the timing.
- It can add to fluid and salt changes caused by water tablets (diuretics) and some other medicines.
- Tell your prescriber about all your medicines and any kidney problems before using it.
Available as: An oral liquid or a powder made up into a drink.
Answers
Magnesium citrate: frequently asked questions
What is magnesium citrate used for?
It is an osmotic laxative used to relieve constipation and to empty the bowel before procedures such as a colonoscopy, by drawing water into the gut.
Why do I need to drink plenty of fluids?
It works by shifting water into the bowel and can cause fluid and salt changes, so drinking plenty of clear fluids helps prevent dehydration.
Who needs to be careful with it?
People with kidney problems and elderly or frail people need extra caution, because they can build up too much magnesium or become dehydrated.
Will it cause diarrhoea?
Yes, diarrhoea is expected, and it is strong and watery at the larger amounts used to prepare the bowel before a procedure.
Can I use it every day for constipation?
No, it is meant for short-term or occasional use rather than daily long-term use, unless your prescriber advises otherwise.
The wider class
About Osmotic laxative (also for bowel preparation)
Magnesium citrate belongs to the osmotic laxative (also for bowel preparation) class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF
- NICE CKS
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