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Epsom salt is magnesium sulfate, and the standard claim is that soaking in it delivers magnesium transdermally to relieve muscle soreness and aid recovery.

The honest position: the evidence for meaningful transdermal magnesium absorption is weak. The stratum corneum is an effective barrier to charged ions, magnesium is a divalent cation, and the studies most often cited are small, uncontrolled, or manufacturer-associated. There is no good evidence that a bath raises serum magnesium.

What is real is simpler and does not require the claim:

• Warm water relaxes muscles and increases local blood flow. That is the bulk of the benefit and it happens in a plain bath.
• Salt water is mildly osmotic, which can reduce local swelling.
• Magnesium sulfate softens water and changes the feel of a bath.
• The ritual itself — twenty minutes horizontal, warm, unreachable — has a real effect on how people feel, and there is nothing dishonest about buying it.

Salt	Composition	Note
Epsom	Magnesium sulfate	The standard; cheap by the kilogram
Sea salt	Sodium chloride plus trace minerals	Mildly drying at high concentration
Dead Sea salt	High in magnesium, potassium, calcium chloride	Has genuine supportive evidence in psoriasis, alongside sunlight — balneotherapy
Himalayan pink salt	Sodium chloride with iron oxide traces	The pink is iron. The "84 trace minerals" claim describes contaminants at trace levels
Sodium bicarbonate	—	Softens water, soothes itch; the basis of an oatmeal-and-bicarb soothing bath

• Salt is drying at concentration. A heavily salted bath draws water out of the skin osmotically, which is the opposite of what dry or eczematous skin needs. Use less, keep it short, and moisturise afterwards.
• Not on broken skin or open wounds — it stings, and it is not sterile.
• Warm, not hot, and 15–20 minutes. Long hot baths strip lipids.
• Rinse afterwards and apply a Body Lotion to damp skin.
• Undissolved salt scratches, so dissolve it under running water before getting in.
• Cardiovascular caution: very hot baths cause vasodilation and a blood-pressure drop, and standing up afterwards can cause fainting. This matters for older people, pregnant people and anyone on antihypertensives.

"Bath salts" was adopted as a street name for a class of synthetic cathinone stimulant drugs sold in the early 2010s, packaged deceptively and labelled "not for human consumption". They have nothing whatsoever to do with the bathing product, and the association is purely a marketing deception by drug sellers. Cosmetic bath salts are mineral salts.

Cosmetic in the US — FDA primary category (02) bath preparations — with no premarket approval. Colour additives must be approved for the intended use, and certified colours must come from FDA-certified batches.

The claims are the regulatory pressure point: "relaxes muscles" is a structure-function claim and, read strictly, is a drug claim under FD&C Act sec. 201(g). "Soothing", "relaxing bathing experience" and similar sensory wording stays cosmetic, which is why packaging uses it.

In the EU, permitted colorants come from the Annex IV closed positive list of 153 entries, and named fragrance allergens must be declared above 0.01% in rinse-off products.

Two to three years or more. Dry salts are essentially indefinite; moisture is the only enemy, and a caked or hardened jar has been damp — usually from being left open in a bathroom. That is a texture problem rather than a safety one.

Fragrance fades over time, and any added botanicals will eventually mould if the salt takes up water. Keep it sealed and out of the steam.

• Bath and Body — the shelf this sits on, and the rest of it.
• Cosmetics — what the label is legally allowed to say, and the full catalogue.
• The rest of Soaking — Bubble Bath · Bath Oil · Bath Bomb · Bath Capsule · Bath Milk · Shower Steamer.
• Sea Salt Spray — the same osmotic effect on hair.
• Regulatory statements here follow the FDA and EU sources listed on Cosmetics, read 16 August 2026.