Epsom Salt Baths for Sore Muscles: The Test That Hid the Salt

The salt-isolating trial. Byerley NA. The Effect of Magnesium Sulfate on Delayed Onset Muscle Soreness. Master of Science thesis, Texas State University-San Marcos, 2010. Handle: hdl.handle.net/10877/9523. Not peer reviewed; no funder or sponsor declared. 26 physically active high-school students (14 male, 12 female; 16.04 ± 1.08 years) did eccentric biceps curls with the non-dominant arm (5 × 10 at 120% and 2 × 10 at 100% of estimated one-repetition maximum) and were allocated by sign-up order to no treatment (n = 8), a 20-minute arm soak in a 15-gallon whirlpool at 43–45 °C (n = 9) or the same soak with 1% Epsom salt, about 600 g per 15 gallons (n = 9). Measures were taken at 0, 24, 48 and 72 hours, with the soaks given after the measurements. The soakers were not told which water they had and were watched so they could not taste it; the untreated group knew it was untreated. Salt vs no salt: no difference in pain (Graphic Pain Rating Scale, p = 0.804) or in any other measure. Soak vs no soak: across the follow-up, pain (Graphic Pain Rating Scale, 0–24, higher = more pain) differed from no treatment for both soaks (control vs hot water p = 0.002; control vs Epsom p = 0.007), with no significant difference before 72 hours; at 72 hours the scores were 16.88 without treatment, 6.44 with hot water and 7.78 with Epsom hot water. Disability (DASH, 11–55) differed from no treatment only for plain hot water (p = 0.027); at 72 hours the scores were 29.38, 15.22 and 18.56. The overall multivariate test was not significant (F(14,36) = 1.58, p = 0.132); groups fell short of the planned 15; the pain scale was modified by the author; no blood or urine magnesium was measured. The author notes that the hot-water pain result conflicts with two earlier trials (Kuligowski et al. 1998; Vaile et al. 2008), and the thesis took the 1% concentration and the skin-absorption rationale from the Waring report below.

Where the absorption claim comes from. Gröber U, Werner T, Vormann J, Kisters K. Myth or Reality—Transdermal Magnesium? Nutrients 2017;9(8):813. doi:10.3390/nu9080813. Narrative review; the authors declare no conflicts of interest. The review describes a report by Waring (University of Birmingham) in which 19 people took full-body baths in Epsom salt at 50–55 °C for 12 minutes a day for 7 days. The report claimed that plasma magnesium rose from 104.68 to 114.08 ppm/mL after the first bath and to 140.98 ppm/mL after the seventh, and urine magnesium from 94.81 to 198.93 ppm/mL. The reviewers rate it "of similar questionable quality" and note that it appeared only on the Epsom Salt Council's commercial website, never in a peer-reviewed journal. Two controlled tests they review found no rise in blood magnesium: a randomized, placebo-controlled trial of a magnesium-sulphate lotion (34 volunteers, 10 mL three times a day for three days) showed no difference in serum magnesium, and eight people immersed for two hours in Bath spa water at 35 °C showed no change in plasma magnesium (O'Hare et al. 1985, BMJ; a spa-water study, not an Epsom bath). Hair follicles and sweat glands make up only 0.1–1% of the skin surface. The review does not claim that nothing can cross: daily hypertonic Dead Sea bathing raised some serum minerals in psoriatic patients, and a magnesium cream produced a small serum rise that was statistically significant only in a subgroup of 20 non-athletes; higher concentrations are untested. The reviewers' verdict: the propagation of transdermal magnesium is "scientifically unsupported".

Magnesium gel. Coates AM, Patel DV, Binet ER, Gibala MJ. No Effect of Topical Application of a Commercial Magnesium Gel on Exercise Recovery in Active Individuals. Int J Sport Nutr Exerc Metab 2025;35(5):416–423. doi:10.1123/ijsnem.2025-0034; PMID 40537122. Abstract only; independence undetermined. Randomized, double-blind, parallel design: 35 healthy active people ran downhill for 40 minutes, with magnesium or placebo gel applied to each thigh 10 minutes before and immediately after. Soreness (0–100 visual analogue scale after a sit-to-stand), peak isometric knee-extensor strength, plasma creatine kinase and serum IL-6 measured before, immediately after, and 24 and 48 hours after: no differences between treatments on any outcome. The authors note the dose (0.53 mg/ml) or application protocol may have been insufficient.

Warm water in the pooled evidence. Wang Y, Lu H, Li S, et al. Effect of Cold and Heat Therapies on Pain Relief in Patients with Delayed Onset Muscle Soreness: A Network Meta-Analysis. J Rehabil Med 2022;54:331. doi:10.2340/jrm.v53.331; PMID 34636405; PROSPERO CRD42020170632. Independent (research grants; no conflicts declared). 59 RCTs, 1,367 people, 10 cold and heat treatments. Hot/warm-water immersion: SUCRA 20.6% (mean rank 8.1) at 24 hours, 36.5% (6.7) at 48 hours and 32.2% (7.1) beyond 48 hours, placing ninth, seventh and eighth of ten; cold-water immersion placed sixth at all three time points. Scope: adults over 18 with delayed-onset soreness, treatment within one hour of exercise, pain scores only. The pooled trials contain no Epsom salt or magnesium arm. The authors rate the included studies of limited quality, most with weak blinding or allocation concealment, and call for better-designed trials; rankings are not effect sizes.

Warm-water subgroup. Dupuy O, Douzi W, Theurot D, Bosquet L, Dugué B. Front Physiol 2018;9:403. doi:10.3389/fphys.2018.00403. Meta-analysis of 99 studies. Immersion subgroups for delayed-onset soreness: 15 °C or colder, g = −0.62 (95% CI −0.93 to −0.32; 234 participants, 23 groups), significant; 36 °C or warmer, g = 0.53 (95% CI −0.44 to 1.51; 3 groups, 30 participants), not significant, which is a null result rather than evidence that warm water worsened soreness. Massage produced the largest reduction in delayed-onset soreness (g = −2.26). The authors note that the placebo effect of recovery techniques could not be eliminated.

Heat without salt. Wellauer V, et al. PLOS One 2025;20(5):e0322416 (doi:10.1371/journal.pone.0322416), cold vs hot water immersion in women: "neither improved subjective or objective recovery outcomes during the 72 h follow-up compared to CON in women following a muscle-damaging exercise protocol." Ahokas EK, et al. Biol Sport 2023;40(3):681–689 (doi:10.5114/biolsport.2023.119289): an infrared sauna session lowered muscle soreness compared with passive rest right after recovery and 14 hours later (p = 0.003 and p = 0.005); no placebo was possible. Ahokas EK, et al. Sports Med Open 2025;11:106 (doi:10.1186/s40798-025-00910-0), systematic review: acute effects of post-exercise heat on performance recovery were uncertain (no effect in 4 studies, benefit in 4, adverse in 1), with subjective benefits such as perceived sleep quality and recovery; evidence quality low to moderate.

Float-tank trials. None of the three measured magnesium or separated the salt from the water, warmth, darkness and quiet, and none attributed its soreness result to the salt; the salt's stated role was buoyancy. Caldwell LK, Kraemer WJ, Post EM, et al. Med Sci Sports Exerc 2022;54(8):1371–1381. doi:10.1249/MSS.0000000000002906 (accepted manuscript; foundation and university funding, no conflicts declared): 11 resistance-trained men did six sets of 10 back squats at 80% of one-repetition maximum, then spent one hour floating in a dark, silent tank of about 300 gallons holding more than 1,200 lb of Epsom salt at 34.44 °C, or one hour in a reclining chair in a lit room watching a nature documentary. Soreness was lower across recovery (p = 0.035), most of all right after the session (36% lower; effect size 1.3); pain intensity, muscle-damage and inflammation markers, and jump performance were unchanged. The authors credit the reduced sensory environment and relaxation and note that only large effects were detectable with 11 people. Morgan PM, Salacinski AJ, Stults-Kolehmainen MA. J Strength Cond Res 2013;27(12):3467–3474. doi:10.1519/JSC.0b013e31828f277e (unfunded): 24 untrained men did 50 maximal eccentric contractions of the non-dominant knee extensors and flexors, then floated for one hour or sat for one hour in the laboratory with phones and homework allowed. No difference in delayed-onset soreness; knee-extension pain was 0.57 lower on a 0–10 scale immediately after the float only (p = 0.011); knee-extensor force was 8% lower immediately after the float; 14 of 24 fell asleep in the tank. The authors attribute the pain drop to relaxation and raise absorption of magnesium sulfate across the skin only as an untested possibility; they measured neither magnesium nor inflammation. Broderick VL. The effects of flotation-restricted environmental stimulation therapy on post-exercise recovery in trained athletes. Master of Health, Sport and Human Performance thesis, University of Waikato, 2019. Handle: hdl.handle.net/10289/13557. Not peer reviewed; independence undetermined. In a crossover design, 19 trained male team-sport athletes completed a running-based exercise simulation plus walking lunges and a wall-sit, then floated for 45 minutes in Epsom-salt water at 35 °C, with about 15 more minutes to shower and dress, or sat for 60 minutes in a dim 21 °C room without devices. Perceived muscle soreness was lower after the float (condition effect p = 0.002), by 1.55, 1.84 and 2.11 points on a 0–10 scale relative to the comparison straight after recovery and 12 and 24 hours later. A placebo effect could not be ruled out, and the thesis suggests comparing the float with a warm bath to find the mechanism.

Swallowed magnesium. Tarsitano MG, et al. Effects of magnesium supplementation on muscle soreness in different type of physical activities: a systematic review. J Transl Med 2024. doi:10.1186/s12967-024-05434-x. 4 eligible studies of 1,254 identified; 73 participants (60 men, 13 women) aged 19–27; capsules in 3 of 4. FitChef's claim verdict: "four studies is a signal, not a consensus". The evidence covers magnesium taken by mouth and does not transfer to baths, gels or sprays.

Dosing, scope and independence. No located trial tested an amount of salt, a water temperature or a soak time for soreness beyond the single 1% arm-soak comparison, so no dosing advice is evidence-based. Scope: post-exercise soreness in healthy exercisers. Lactic acid: Schwane et al. 1983 (doi:10.1080/00913847.1983.11708485; abstract only) found lactic acid unrelated to delayed-onset soreness. Independence: Wang et al. and Morgan et al. independent; Caldwell et al. independent of the float industry and Epsom-salt sellers; Byerley independent of industry but not peer reviewed; Gröber et al. declare no conflicts of interest; Coates et al. and Broderick undetermined; the Waring report is industry-hosted and was never peer reviewed.