Does a magnesium bath help sore muscles?

The short answer: no good evidence yet

Bottom line: we found no randomized trial testing a magnesium or Epsom salt bath for sore muscles. Our search coverage was limited, so treat this as a provisional read rather than a final verdict. The evidence for bath-specific claims is very low.

Epsom salt, which is magnesium sulfate, is a common addition to baths for achy muscles, and the claim shows up everywhere. The research behind it is thin. Examine.com's magnesium overview notes a lack of evidence that magnesium is absorbed through the skin, so any soak-related relief is probably not from the magnesium.

Does magnesium actually get through your skin?

Bottom line: probably not in amounts that matter, and this has not been shown in people. Cleveland Clinic on Epsom salt states that no clinical trials have confirmed the benefits of Epsom salt baths, and that no definitive studies show enough magnesium passes through the skin to correct a deficiency.

The idea that dissolved magnesium and sulfate enter the body through the skin is a theory. It has not been demonstrated in people soaking in a bath.

The closest human data is a pilot study of transdermal magnesium cream. It enrolled 25 healthy adults, randomized them to magnesium or placebo cream, and measured serum and urinary magnesium. We could not access its results, so we cannot report effect sizes. A cream is still not a bath, so we treat it as indirect evidence.

A PubMed-listed review on transdermal magnesium notes that transdermal products are heavily marketed, while oral supplementation is the well-documented route. We only had its reference listing, so we use it as background, not proof.

What the bath study actually measured

Bottom line: the one bath study we found measured fluid loss, not muscle pain.

A hot water immersion study in mixed martial arts athletes compared immersion with and without about 2 kg of Epsom salt in 125 L of water, roughly 1.6 percent. Body mass loss was similar either way, so the salt added no measurable effect on weight loss.

The study did not measure soreness, recovery or absorption. Its relevance to sore muscles is indirect at best.

What about muscle cramps?

Bottom line: the magnesium evidence for cramps comes from oral and IV dosing, not baths.

A review of magnesium for skeletal muscle cramps describes an ongoing trial of a transdermal magnesium spray in dialysis patients. It reports one trial with a 79 percent reduction in cramps in the magnesium group versus 32.4 percent with placebo. We could not confirm the details behind those figures, so we treat them as unverified.

Safety: Epsom salt is not harmless

Bottom line: swallowing Epsom salt can be dangerous, and kidney problems raise the stakes.

A case report of near-fatal hypermagnesemia describes a patient with normal kidney function who took Epsom salt orally and reached a serum level of 8.71 mmol/L, which required dialysis. That was ingestion, not a bath, but it shows magnesium sulfate can become dangerous when levels rise.