Magnesium absorption decoded: transdermal sprays vs oral salts — and why bulk flakes beat branded oil

Wellness shelves sell ‘clinical magnesium’, ‘Zechstein oil’ and ‘sleep blends’ as if they share one molecule. They do not. The supplement facts panel (or SDS for bath flakes) names a magnesium compound — chloride, citrate, bisglycinate, oxide — and a delivery story: spray, capsule, powder drink or tablet.

Absorption and gut tolerance follow the chemistry. Price usually follows the bottle, the perfume and the influencer, not the Mg²⁺ ion. DupeStack teardowns strip that packaging tax back to bulk salts.

BetterYou Magnesium Oil Spray is roughly 31% magnesium chloride in water. The ‘oil’ label is marketing: you are spraying a strong brine. The same Dead Sea / Zechstein-style hexahydrate flakes (CAS 7791-18-6) dissolve at about 300 g per litre for a DIY concentrate — see the BetterYou teardown for the cost-per-litre maths versus a 100 ml branded atomiser.

Transdermal magnesium is popular because it ‘avoids the gut’. Charged Mg²⁺ does not freely cross intact skin like a steroid cream. Some local tingling is the brine on sweat ducts and abraded skin, not proof of systemic loading. Patch-test; rinse if it burns; keep off broken skin. Food-supplement / topical mineral use — not a treatment for cramp, migraine or deficiency without a clinician.

Oral stacks live or die on elemental magnesium milligrams — the Mg atom mass on the label — not the milligrams of ‘magnesium bisglycinate compound’. Oxide is cheap and poorly absorbed. Citrate is inexpensive and osmotic (Beam Dream often leads with citrate plus cocoa). Bisglycinate is a chelated salt many people tolerate better at night — Nutrition Geeks capsules and the Beam dupe stack both sit in that family.

Typical food-supplement evening pattern discussed on our teardowns: on the order of 200–300 mg elemental Mg with food, reduced if stools loosen. Capsules win on convenience; unflavoured bisglycinate powder wins on cost per milligram. Neither replaces GP advice if you take diuretics, heart medicines or have kidney disease.

A 100 ml ‘oil’ at ~£13 is a tiny volume of chloride solution. A 1 kg bag of flakes makes roughly three litres of spray-strength brine at a few pounds — the BetterYou teardown puts that near 79% savings on working-litre maths. You pay for the glass bottle and the Zechstein story, not a proprietary molecule.

The same logic hits oral tubs: Beam’s cocoa ritual and Nutrition Geeks’ ‘3-in-1’ extras are delivery and marketing. Buy the salt you intend to use, weigh elemental Mg once with a 0.1 g scale, and ignore the sleep-medicine claims on either pouch.

Comparing a BetterYou spray to Beam Dream or Nutrition Geeks capsules without naming the salt is category error — chloride topical versus citrate/glycinate oral. For drinkable sodium/potassium with a little magnesium, see the LMNT electrolyte teardown instead of doubling up random Mg products.

Berocca-style effervescents also carry magnesium among B-vitamins — useful as a tablet cost check, not as a transdermal substitute. Always one chemistry decision at a time; this site compares food supplements, not medicines.