Magnesium Side Effects and Interactions
Magnesium supplements are widely used and, at labelled doses in people with healthy kidneys, generally well tolerated. The considerations worth knowing about fall into three groups: digestive effects, medication interactions, and the situations where supplementing is not appropriate without medical advice.
Digestive effects
The most commonly reported effect is on the bowel. Magnesium salts draw water into the intestine, which is the mechanism behind their deliberate use as laxative preparations at higher doses. At supplement doses the effect varies between people and between forms, and it is the usual reason people switch form or split a dose.
Medication interactions
These are the ones to raise with a pharmacist, because several are common medications and the fix is usually spacing rather than avoidance.
- Certain antibiotics — tetracyclines and quinolones can bind magnesium in the gut, reducing absorption of the antibiotic.
- Bisphosphonates — absorption is reduced when taken together.
- Diuretics — different classes affect magnesium retention in opposite directions.
- Proton pump inhibitors — long-term use has been associated with low magnesium levels.
Kidney function
This is the most important single point on the page. Excess magnesium is cleared by the kidneys. Where kidney function is reduced, magnesium can accumulate, and at high levels this is a serious clinical problem rather than a minor side effect. Anyone with chronic kidney disease should not take magnesium supplements except on medical advice.
Signs to stop and seek advice
Persistent nausea, unusual muscle weakness, marked drowsiness, or an irregular heartbeat after starting a magnesium supplement warrant stopping the product and contacting a healthcare professional. In an emergency, contact your local emergency services.
Reduce the variables
If you are introducing magnesium, change one thing at a time: one product, at the labelled dose, long enough to see how you tolerate it. Adding a magnesium supplement on top of a multivitamin and an electrolyte drink makes it impossible to attribute anything, and can quietly stack the dose — see magnesium dosage.
This page is consumer information about product labelling, not medical advice. Please read our medical disclaimer.
References
- NIH Office of Dietary Supplements — Magnesium (Health Professional Fact Sheet)
- NHS — Vitamins and minerals: Others