BMA Concise Guide to Medicine & Drugs

Magnesium Hydroxide

Brand names Cream of Magnesia, Milk of Magnesia

Used in the following combined preparations Carbellon, Maalox, Milpar, Mucogel, and others


Drug group Antacid and laxative

Overdose danger rating Low

Dependence rating Low

Prescription needed No

Available as generic Yes


Magnesium hydroxide is a fast-acting antacid given to neutralize stomach acid. The drug is available in a number of over-the-counter preparations for the treatment of indigestion and heartburn. Magnesium hydroxide also prevents pain caused by stomach and duodenal ulcers, gastritis, and reflux oesophagitis, although other drugs are normally used for these problems nowadays. It is also used as a laxative; it works by drawing salt and water from the wall of the bowel to soften the faeces.

Magnesium hydroxide is not often used alone as an antacid because of its laxative effect. However, this effect is countered when the drug is used in combination with aluminium hydroxide, which can cause constipation.


Follow instructions on the label. Call your doctor if symptoms worsen.

How taken/used Tablets, liquid, powder.

Frequency and timing of doses Antacid 1–4 x daily as needed with water, preferably an hour after food and at bedtime. Laxative Once daily, at bedtime.

Adult dosage range Antacid 1–2g per dose (tablets); 5–10ml per dose (liquid). Laxative 30-45ml per dose (liquid).

Onset of effect Antacid within 15 minutes. Laxative 2–8 hours.

Duration of action 2–4 hours.

Diet advice None.

Storage Keep in original container at room temperature out of the reach of children.

Missed dose Take as soon as you remember.

Stopping the drug When used as an antacid, can be safely stopped as soon as you no longer need it. When given as ulcer treatment, follow your doctor’s advice.

Exceeding the dose An occasional unintentional extra dose is unlikely to be a cause for concern. But if you notice any unusual symptoms, or if a large overdose has been taken, notify your doctor.


Diarrhoea is the only common adverse effect of magnesium hydroxide. Dizziness and muscle weakness due to absorption of excess magnesium may occur in people with poor kidney function (see Prolonged use).


General note Magnesium hydroxide interferes with the absorption of a wide range of drugs taken by mouth, including tetracycline antibiotics, iron supplements, diflunisal, phenytoin, gabapentin, and penicillamine, therefore you should allow 1–2 hours between magnesium hydroxide and other medications.

Enteric-coated tablets As with other antacids, magnesium hydroxide may allow break-up of the enteric coating of tablets, sometimes leading to stomach irritation.


Be sure to consult your doctor or pharmacist before taking this drug if:

· You have a long-term kidney problem.

· You have a bowel disorder.

· You are taking other medicines.

Pregnancy No evidence of risk, but discuss the most appropriate treatment with your doctor.

Breast-feeding No evidence of risk, but discuss the most appropriate treatment with your doctor.

Infants and children Not recommended under 3 years except on the advice of a doctor. Reduced dose necessary for older children.

Over 60 No special problems.

Driving and hazardous work No known problems.

Alcohol Avoid excessive alcohol as it irritates the stomach and may reduce the benefits of the drug.


Magnesium hydroxide is for occasional use and should not be taken for prolonged periods without consulting your doctor, especially if you experience persistent abdominal pain while taking the drug. If you are over 40 years of age and are experiencing long-term indigestion or heartburn, your doctor will probably refer you to a specialist. Prolonged use in people with kidney damage may cause drowsiness, dizziness, and weakness, resulting from accumulation of magnesium in the body.