The most common side effects of magnesium supplements are stomach upset, nausea, vomiting and diarrhea (as nearly all forms of magnesium have a laxative effect). Magnesium hydroxide (milk of magnesia) and magnesium citrate, for example, are commonly found in over-the-counter products to treat constipation. Fortunately, if you are taking magnesium for a reason other than its laxative effect — i.e. because you are not getting adequate magnesium from your diet, there are forms which are less likely to cause diarrhea.
Although it's important to get adequate amounts of magnesium, be aware that taking excessive amounts of magnesium from supplements or over-the-counter laxatives and antacids can cause elevated blood levels of magnesium, which can lead to low blood pressure, drowsiness, muscle weakness, slowed breathing, and even death. Dangerous blood levels of magnesium have also been reported in people ingesting or gargling with large quantities of Epsom salt (magnesium sulfate).
People with kidney disease should not take magnesium without physician supervision. Magnesium can also interact with a number of medications, including certain blood-thinning and antidiabetes drugs, and should be taken at least two or more hours apart from other common medications, including certain statin drugs and antibiotics. Some antacid medications can decrease magnesium's laxative effect — something to keep in mind if you are taking magnesium for this purpose.
Topical magnesium products such as magnesium creams, sprays and oils can cause skin irritation.
For more details about magnesium side effects, potential drug interactions, and other concerns, see the Concerns and Cautions section of the Magnesium Supplements Review.