#12 Magnesium for Leg Cramps

Night time leg cramps are very common and can ruin sleep. One study surveyed 490 veterans and found that 60% of them had suffered from night time leg cramps and 1/4 of those had them regularly. A lot of people use magnesium to ease their nocturnal cramps and they often swear by them, but do they actually work? This week, we delve into the evidence of magnesium for leg cramps.

A study published in JAMA in May of 2017 has tried to answer this question.


This was a randomised, double blind, placebo controlled trial that was conducted in Israel. They first recruited people using flyers and posters inviting anyone who suffers from leg cramps. Then, for anyone who expressed interest, they put them through a 2 week screening phase where they didn’t give them any treatment but asked them to keep a diary where they had to document how many night time cramps they got and the nature of these cramps. They then used this diary to include or exclude people based on whether they matched the criteria. The criteria was that they had to have at least 4 episodes of nocturnal leg cramps during this 2 week screening phase. And the criteria for what they called a cramp was fairly strict: it had to be painful, occurring at rest, involuntary and causing a palpable knot in the muscle. They excluded pregnant women and those with renal failure.

166 people responded to the ads, and 94 of those passed the 2 week screening phase and made it into the trial. On average these people were getting 8 night time leg cramps per week. 60% of them were women, and the average age was was 65.

They were randomised, with concealed allocation, to receive either 865mg of magnesium oxide daily (520mg of elemental magnesium) or placebo to take at night before bed.

The primary outcome was the change in the number of cramps per week. Secondary outcomes included severity and duration of the cramps, quality of life and quality of sleep. The way they got their data was again with that diary –  Every morning when they woke up they’d write down how many episodes they had, how severe it was from 1-10 and how long they went for. The investigators sent them a text message every morning to remind them to fill out their diary and investigators called them twice per week to see whether there were any adverse effects, to make sure they were taking the magnesium or the placebo and to make sure they were filling out their diary.

They followed them up for 4 weeks and here’s what they found:


Magnesium was able to reduce the number of cramps – there were 3.41 less cramps per week when the participants were taking magnesium. Coming from an average of 8 cramps per week that’s about a 40% reduction in the number of cramps. Which is pretty impressive.

The problem is though….that in the placebo group there were 3 less cramps per week. Which is also pretty impressive. And while magnesium was able to reduce cramps by 0.4 episodes per week over placebo, this was not statistically significant.

Now this doesn’t necessarily mean it isn’t true. It wasn’t powered with enough people to detect such a small difference. They designed the trial to have enough power to detect a difference of 1 episode of cramping per week, so they would have needed 2-3 times more people to accurately say whether this reduction of 0.4 episodes per week was true. But even if it is true is a reduction of 0.4 nocturnal leg cramps per week a significant one? This equates to 1 less cramp every 2-3 weeks. So in someone getting 8 cramps per week is this meaningful? I don’t think so.

They also presented the results in a dichotomous way which is what you need to get a number needed to treat. They defined a “minimum clinically important difference” as a reduction in leg cramps by 25%. They found that in the placebo group, 63% of them reached this minimum clinically important difference, but in the magnesium group, 75% of them reached it. Again, this was not statistically significant, the P value was 0.21, but again, that doesn’t mean it isn’t true – it just means that there is a 20% probability that these results occurred by chance.

Therefore, in this study, 12% more people obtained a minimum clinically important improvement in their symptoms in the magnesium group compared to placebo. If this is true, it means the number needed to treat for magnesium over placebo is around 9.

There was no difference between magnesium and placebo for any of the secondary outcomes, like quality of life or sleep quality.

In terms of side effects, there was no difference between magnesium and placebo. But other studies have shown increased rates of diarrhoea with magnesium.

Other Evidence

There have been 4 other randomised controlled trial looking at magnesium for idiopathic leg cramps (Cochrane), and they all showed pretty much the same thing. That amazingly, there was a great improvement with placebo, but no greater improvement with magnesium. 1 of the trials also showed this very slight improvement with magnesium over placebo but it also was not statistically significant.

There have also been three trials looking at using magnesium specifically in pregnant women and while one found it was better than placebo, the other 2 didn’t.

Bottom Line

In those who suffer nocturnal leg cramps, placebo and magnesium are very affective at reducing the number of cramps, reducing them by almost half. There was a non-statistically significant improvement with magnesium over placebo which, if true, equates to a reduction in the number of leg cramps by 0.4 episodes per week or a number needed to treat of 9 for a minimum clinically important improvement (P=0.21).

So should we stop using magnesium? Well, given that the placebo response is so good for this condition, and given there is absolutely no other effective treatment for nocturnal leg cramps and given that the side effects are minimal….perhaps magnesium is a reasonable option. What are your thoughts? Leave a common

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