Magnesium supplementation that is not excessive is well tolerated and probably won’t cause side effects, and it’s difficult to ingest too much magnesium from food sources alone. Excess magnesium in the body is eliminated by the kidneys, so the risk of magnesium toxicity from food is quite low for healthy people.[1]
Supplementing with high doses and certain magnesium salts (i.e., magnesium carbonate, magnesium chloride, magnesium gluconate, magnesium hydroxide, and magnesium oxide) can have a laxative effect — though some people choose to take certain forms of magnesium for this reason.[2][3] Unabsorbed magnesium salts have an osmotic effect in the intestinal tract and can increase gastric motility. In one study, 12% of participants experienced diarrhea with a 1,000 mg dose of magnesium oxide,[4] but this form of magnesium is poorly absorbed, and the dose is well above the tolerable upper limit (UL) for magnesium of 350 mg for healthy adults.[5] Nausea, diarrhea, and abdominal cramping are also occasional side effects reported from supplemental magnesium.
Magnesium supplements can also interfere with the absorption of certain medications, including bisphosphonates (used for treating osteoporosis) and antibiotics — to avoid these interactions, individuals who are taking these types of medications should talk to their healthcare provider about how to space out and time their supplemental magnesium.[3]