A recent study published in Cochrane Database of Systemic Reviews assessed the effects of magnesium supplementation compared to no treatment, placebo control or other cramp therapies in individuals with skeletal muscle cramps. 

The findings indicated that for idiopathic cramps—largely in older adults (mean age 61.6 to 69.3 years) presumed to have nocturnal leg cramps—differences in measures of cramp frequency when comparing magnesium to placebo were small, not statistically significant, and showed minimal heterogeneity. The percentage of individuals experiencing a 25% or better reduction in cramp rate from baseline was also equivalent. Similarly, no statistically significant difference was found at four weeks, in measures of cramp intensity or cramp duration. Additionally, the single study comparing magnesium to no treatment failed to find statistically significant benefit on a three-point ordinal scale of overall treatment efficacy. 

In conclusion, it was stated that magnesium supplementation most probably does not provideclinically meaningful cramp prophylaxis to older adults experiencing skeletal muscle cramps. In contrast, for those experiencing pregnancy-associated rest cramps the literature is conflicting and further research in this population is needed. 

Source: Cochrane Database of Systemic Reviews. 2020 Sep 21;9:CD009402.doi:10.1002/14651858.CD009402.pub3.