How Much of Milk of Magnesia Is Safe for the Quick Relief of Severe Constipation?

Milk of magnesia, otherwise known as magnesium hydroxide, is an over-the-counter medication that's available in liquid or tablet form. It is most often prescribed as a treatment for acute constipation. Though it is a common treatment, it does come with some possible risks related to dosage levels. According to the National Digestive Diseases Information Clearinghouse, you should limit the amount of milk of magnesia that you take to reduce the risk of side effects such as electrolyte imbalances.

How It Works

Milk of magnesia is a saline-based laxative that can treat constipation, as long as there's no indication of a bowel obstruction. The saline in the milk of magnesia works like a sponge when it hits the colon, drawing water into the area to soften the stool and ease its passage. Taking too much milk of magnesia can result in watery diarrhea, so you should always follow dosage instructions carefully.


Adults should take milk of magnesia once a day, just before bedtime, to ease acute constipation. Milk of magnesia takes several hours to start working, so taking it before bed means that you'll likely have a bowel movement in the morning. If you do not have a bowel movement, you may need to slightly increase your dose over the course of several days until you find an amount that works.


Adults should start by taking 2 tbsp. of milk of magnesia once daily. It is important to measure the dose carefully to avoid diarrhea. If 2 tbsp. does not work, increase the dosage amount by 1 tsp. per dose until you reach 3 tbsp. Do not exceed 3 tbsp. per day without checking with your doctor.


Milk of magnesia is not appropriate for long-term or repeated relief of constipation. This is because the saline in milk of magnesia can cause dehydration and electrolyte deficiency, especially in young children and the elderly. Check with your doctor before using milk of magnesia to treat constipation, and inform him if your symptoms do not improve. Constant constipation can be a sign of a more serious condition, such as a bowel obstruction.

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