Magnesium for Muscle Cramps: Read This Before You Buy
Magnesium for cramps is one of the best-selling ideas in the supplement aisle, and one of the worst supported. A 2020 Cochrane review — about as rigorous as medical evidence gets — concluded that magnesium supplements are unlikely to be effective for common (idiopathic) muscle cramps, at any dose and in any form tested. For pregnancy-related leg cramps the evidence is conflicting and very low certainty, so no firm conclusion can be drawn either way.
You would never learn any of that from the packaging. If your cramps stem from a diagnosed magnesium deficiency, correcting it makes sense. For ordinary night-time or exercise cramps, the research says magnesium probably isn’t your answer, and we’d rather lose the sale than pretend otherwise.
A better first step: if cramps are frequent, severe, come with muscle weakness, or started after a new medication, see your doctor. Some causes are treatable, and that's worth more than any supplement.
Sources: Garrison et al., Magnesium for skeletal muscle cramps, Cochrane Database of Systematic Reviews, 2020 (https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD009402.pub3/full)
This article is general information, not medical advice.