Does magnesium prevent muscle cramps?

Cramps in older adults

The 2020 Cochrane review found 5 trials in people with cramps without a known cause. Most were older adults, with a mean age of 62 to 69, who had night-time leg cramps.^[1] After 4 weeks:

• The number of cramps per week was 0.18 lower than with placebo, a difference that could be due to chance (5 trials, 307 people).^[1]
• The share of people whose cramps fell by at least 25% was about the same in both groups. The certainty of this result was high.^[1]
• How strong the cramps were and how long they lasted did not differ.^[1]

In a trial in Israel, 94 adults with a mean age of 65 took magnesium oxide or a placebo at bedtime for 4 weeks.^[3] Cramps fell from 7.84 to 4.44 a week with magnesium, and from 8.51 to 5.48 a week with placebo. The difference between the groups was 0.38 cramps a week.^[3] The authors judged the fall in both groups to be probably a placebo effect.^[3]

A later trial in Ukraine gave 226 mg of magnesium oxide monohydrate or a placebo at bedtime for 60 days. Among the 175 people who finished, the number of cramps fell by 3.4 with magnesium and by 2.6 with placebo, a significant difference. Sleep quality also improved more with magnesium.^[4]

Cramps in pregnancy

A 2020 Cochrane review of leg cramps in pregnancy found 8 small studies with 576 women. The studies measured cramps in different ways, so their results could not be pooled.^[2] For magnesium, the results were inconsistent:^[2]

• In one trial of 69 women, more women had no cramps at all after treatment.^[2]
• Other results showed little or no difference in how often cramps came.^[2]
• The certainty of the evidence was low.^[2]

A 2021 review of 4 trials with 332 pregnant women found that magnesium did not lower how often cramps occurred.^[5]

Cramps during exercise or disease

The Cochrane review found no trials of magnesium for cramps during exercise. It also found no trials for cramps caused by a disease, such as motor neuron disease (ALS), apart from one small, inconclusive trial in liver cirrhosis.^[1]

Why people expect it to work

Severe magnesium deficiency can cause muscle contractions and cramps.^[6] Trials in older adults with ordinary night cramps, however, found no clear effect.^[1]

Side effects

In the Cochrane trials, gut side effects such as diarrhea affected 11% to 37% of people on magnesium, against 10% to 14% on placebo.^[1] Serious side effects were no more common than with placebo.^[1]

Who should be careful

People with kidney disease have a higher risk of magnesium toxicity and should ask a doctor first.^[6] Pregnant women should not go above 350 mg a day from supplements, the US upper limit, without asking a doctor.^[6]