This article is for general information only and is not medical advice. If you have a health condition, take prescribed medication or are pregnant, speak to your doctor or pharmacist before changing your supplements.
The short answer
Magnesium has a calming reputation, and some people find a daytime dose makes them drowsy within an hour or two. For most healthy adults this is mild and is fixed by moving the dose to the evening or lowering it. Marked drowsiness with weakness is different: it can be a sign of too much magnesium in the body, which mainly affects people with kidney problems and needs medical advice.
This article covers drowsiness during the day. If your problem is waking up groggy after a night-time dose, read can magnesium make you tired the next day?
Why magnesium might make you drowsy
It is often taken for exactly this
Magnesium helps nerves and muscles relax, which is why people take it in the evening. Trials of magnesium for sleep are small and the evidence is weak, but many people do report feeling calmer and sleepier after a dose. Taken at breakfast, that same effect is unwelcome.
The form
Magnesium glycinate contains glycine, an amino acid with a calming effect of its own. It is the form people most often tell us makes them sleepy. Sleep and relaxation blends may also contain L-theanine, herbs or melatonin.
The dose
The NHS says 400 mg or less a day from supplements is unlikely to cause harm. It is easy to go over that without noticing if you combine a magnesium tablet with a multivitamin, an electrolyte drink and a spray. Some indigestion remedies and laxatives also contain large amounts of magnesium.
Blood pressure
You may read that magnesium makes people tired by lowering blood pressure. Magnesium does lower blood pressure, but only slightly. A 2016 analysis of 34 trials found an average fall of about 2 mmHg after roughly three months of supplements. That is too small to make most people drowsy. It may matter more if your blood pressure is already low or you take blood pressure medicine, so check with your GP or pharmacist in that case. Dizziness, feeling sick, blurred vision and fainting are the symptoms of low blood pressure to look out for.
Too much magnesium in the body
Healthy kidneys remove extra magnesium, so a true excess is rare. It becomes more likely with kidney disease or with very large doses, usually from laxatives or antacids. The NIH lists the signs as low blood pressure, nausea, flushing, lethargy and muscle weakness, progressing to breathing difficulty and an irregular heartbeat. This needs urgent medical help. It is not something to manage at home.
What to try
- Move it to the evening. Take it with your evening meal so any drowsiness arrives when you want it.
- Lower the dose. Halve it, or take it every other day, and see how you feel after a week.
- Add up all your sources. Include multivitamins, drinks, sprays and any indigestion or constipation remedies.
- Switch form. If glycinate makes you sleepy, try citrate or malate. Citrate is more likely to loosen stools.
- Use a single-ingredient product so you can rule out other calming ingredients.
Don’t drive or use machinery while you feel drowsy.
Older versions of this article suggested taking extra salt to counter the drowsiness. We have removed that advice. There is no good evidence it works, and extra salt raises blood pressure, which is a risk for many people.
Speak to a doctor or pharmacist if
- you have kidney disease or reduced kidney function
- you take blood pressure medicine, heart medicine or water tablets
- the drowsiness comes with muscle weakness, feeling sick, dizziness or fainting
- you have felt tired for a few weeks and can’t explain it
Call 999 if someone has taken a large amount of magnesium and becomes very drowsy, confused or has trouble breathing.
See how magnesium interacts with other nutrients
Magnesium works alongside calcium, potassium, vitamin D and others. Tap magnesium on our free Mineral Interaction Wheel to see what it works with and what it may compete with. For other side effects, read our magnesium side effects guide.
Sources
- NHS. Vitamins and minerals: Others (magnesium).
- NIH Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals.
- Zhang X, et al. Effects of magnesium supplementation on blood pressure: a meta-analysis of randomized double-blind placebo-controlled trials. Hypertension, 2016.
- NHS. Low blood pressure (hypotension).