Magnesium laxatives (magnesium oxide and magnesium hydroxide): uses and cautions

Magnesium is a mineral that the body needs in small amounts, and the NHS says it helps turn food into energy and keeps the parathyroid glands working normally. Magnesium compounds are also used as medicines. Magnesium hydroxide, sold as Magnesium Hydroxide Mixture BP or milk of magnesia, and magnesium oxide are the best known, and they act as antacids at low doses and as laxatives at higher doses.

The NHS describes four main types of laxative: bulk-forming, osmotic, stool-softening and stimulant. The NHS describes osmotic laxatives as bringing more water from the rest of the body into the bowel so that stool is softer and easier to push out. Its page names lactulose and macrogol as examples and does not specifically list magnesium products, so check the leaflet of any product you are considering.

The product information for magnesium hydroxide mixture describes a slow-acting antacid with a mild laxative action, and says the pharmacology is that magnesium salts are poorly absorbed after oral administration, with only about a third absorbed from the small intestine.

The UK product information for magnesium hydroxide mixture lists two uses: relief of flatulence, heartburn, dyspepsia and indigestion, and relief of occasional constipation. For adults, the elderly and children over 12 years it gives one to two 5 ml spoonfuls in a little water when required as an antacid, and 25 to 50 ml in a glass of water at bedtime as a laxative. It is not suitable for children under 12.

The NHS says that for most adults with constipation it is usually recommended to try lifestyle changes first, before laxatives. It says if you are buying laxatives without a prescription it is usually best to try a bulk-forming laxative first, and that osmotic laxatives take 2 to 3 days to work, so magnesium products are not an instant remedy. If you are taking bulk-forming or osmotic laxatives it is important to drink plenty of fluids to avoid dehydration.

The product information says magnesium hydroxide is contraindicated in severe renal failure, acute gastrointestinal conditions and hypersensitivity to magnesium salts. It says to use with caution in the elderly and debilitated, and in people with impaired liver function, and in impaired kidney function because hypermagnesaemia, too much magnesium in the blood, may result. In young children the use of magnesium hydroxide can cause hypermagnesaemia, especially with renal impairment or dehydration.

Symptoms of excess magnesium listed in the product information include flushing of the skin, thirst, respiratory depression, cardiac arrhythmias, coma and cardiac arrest, though these are described as rare with antacid use. The practical lesson is that regular or high-dose use is not appropriate if your kidneys are not working well, which is common in older people, so ask a pharmacist or GP first.

The NHS says to check with a pharmacist or doctor before taking laxatives if you have had an allergic reaction to a laxative, are pregnant or breastfeeding, have a heart condition such as heart failure, have difficulty swallowing, have a digestive condition such as Crohn’s disease or ulcerative colitis, have lactose intolerance, or are taking opioid painkillers.

The NHS also warns that using laxatives too often or for too long can cause dehydration, a bowel blockage and low potassium in the blood, with muscle weakness, tiredness, needing to pee a lot and heart rhythm problems.

Magnesium salts can reduce the absorption of other medicines. The product information lists ACE inhibitors such as enalapril, azithromycin, tetracyclines, quinolone antibiotics, itraconazole, ketoconazole, nitrofurantoin, rifampicin, bisphosphonates, digoxin, gabapentin, phenytoin, lansoprazole, levothyroxine, mycophenolate, iron preparations, rosuvastatin, hydroxychloroquine and penicillamine among others. As a precaution it advises separating drug administrations by approximately 2 to 3 hours.

Concomitant use with sodium polystyrene sulphonate may produce metabolic alkalosis, and alkaline urine from some antacids increases excretion of aspirin. If you take regular medicines, particularly levothyroxine, antibiotics or iron, ask a pharmacist about timing before using a magnesium laxative or antacid.

Pregnancy and breastfeeding: the product information says use should be avoided unless recommended by a doctor, as with other medicines.

Magnesium supplements for diet are a different matter from laxatives. The NHS says adult men need about 300 mg a day and women about 270 mg a day, that you should be able to get all the magnesium you need from a varied balanced diet, and that taking high doses of more than 400 mg for a short time can cause diarrhoea. Having 400 mg or less a day from supplements is unlikely to cause harm.

The NHS says to stop laxatives bought without a prescription when constipation improves, not to take them for longer than 7 days, and to speak to a doctor or pharmacist if three days of use have not helped. The product information says if constipation persists you should consult your doctor.

Constipation that is new, persistent or alternating with diarrhoea, or accompanied by blood in the stool, weight loss or severe pain, needs medical assessment. Your GP can look for causes, including medicines, and recommend longer-term treatment.

Magnesium hydroxide mixture is on the UK general sale list, so it can be bought without a prescription, and pharmacists can advise on suitability. Prescribers and private clinics offer the same options. The most useful steps are lifestyle measures and a review of constipating medicines.