How long does Milk of Magnesia take to work?
For constipation, it takes anywhere from 30 minutes to 6 hours for you to have a bowel movement. However, for reflux, you will usually feel relief after 30 minutes.
What is Milk of Magnesia and what is it used for?
Milk of Magnesia is an antacid medication which can also be used to treat constipation when used in higher doses. The active ingredient in Milk of Magnesia is magnesium hydroxide.
Who is Milk of Magnesia suitable for?
Milk of Magnesia can be purchased over the counter for those aged 12 and over.
You should not take Milk of Magnesia if:
- You have an allergy to any of the ingredients in Milk of Magnesia
- You have severe kidney failure
- You have a severe stomach or bowel condition
You should seek the advice of your GP if:
- You have kidney or liver problems
- You are elderly
- You take other medications - this is because some medications can be affected by magnesium so it is important to check this beforehand
- You are pregnant or breastfeeding
How to take Milk of Magnesia
Milk of Magnesia should be taken orally. You should shake the bottle before preparing your dose.
For adults and children aged 12+ using this medication as a laxative
Take 5-10 5ml spoonfuls (25 - 50ml) in a glass of water at bedtime.
For adults and children aged 12+ using this medication as an antacid
Take 1-2 5ml spoonfuls with water as required, usually before or after meals, or at bedtime.
How to store Milk of Magnesia
- Store below 25 degrees celsius.
- Keep out of direct sunlight.
- Keep out of the sight and reach of children.
- Do not use after the expiry date.
Milk of magnesia generally does not cause any side effects besides occasional nausea, diarrhoea and stomach cramps.
A rare but serious side effect of Milk of Magnesia is hypermagnesemia, which is excess magnesium in the body. Symptoms of hypermagnesemia are nausea, confusion, vomiting, low blood pressure and reduced reflexes.
Milk of Magnesia - Frequently Asked Questions
Can I take Milk of Magnesia every day?
No, you should not take Milk of Magnesia every day. If you have used this medication for 7 days and still have symptoms, you should speak with your GP or pharmacist to assess if there is an underlying cause.
Our Clinical Authors
This page has been written by a registered UK pharmacist and reviewed by our clinical team to ensure it is accurate, clear and reflects current clinical guidance.
Last medically reviewed: 06 July 2026
Last updated: 06 July 2026