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Sweat itself is odourless. Two glands, two kinds of secretion:

• Eccrine glands cover the body and secrete a dilute salt solution for thermoregulation. Essentially odourless.
• Apocrine glands are concentrated in the underarms, the groin and around the nipples, and become active at puberty. They secrete a viscous, protein- and lipid-rich fluid.

The odour is produced by skin bacteria metabolising apocrine secretion — chiefly Corynebacterium species, and to a lesser extent Staphylococcus — releasing volatile short-chain fatty acids, thioalcohols and steroid derivatives. The characteristic sharp underarm smell is largely thioalcohols, produced by corynebacteria from odourless precursors.

So a deodorant has three possible targets: the bacteria, the odour molecules, or perception.

Approach	Ingredients
Reduce bacteria	Alcohol, triclosan (largely abandoned), farnesol, zinc ricinoleate, silver salts
Lower skin pH so bacteria grow less	Citric acid, mandelic acid, sodium bicarbonate (the latter raises pH but disrupts them)
Absorb or bind odour	Zinc ricinoleate, activated charcoal, starches, zeolite
Mask	Fragrance
Absorb moisture	Starch, silica — but not blocking the gland

Baking soda deodorants are effective and are also the leading cause of the "natural deodorant rash": sodium bicarbonate is strongly alkaline, and repeated application to a warm, occluded, frequently-shaved area disrupts the acid mantle and produces irritation, darkening and sometimes a genuine contact dermatitis.

If a natural deodorant is causing a rash, the bicarbonate is the first suspect. Magnesium hydroxide, zinc ricinoleate and acid-based formulas achieve similar results without the alkalinity.

The "detox period" some brands describe when switching from antiperspirant has no established basis; what is real is that the underarm microbiome shifts when aluminium salts are stopped, and odour can be worse for a couple of weeks before it settles.

The claim that antiperspirants cause breast cancer or Alzheimer's disease is not supported by the evidence, and major cancer research bodies say so. Anyone choosing deodorant over antiperspirant is entitled to; the reason should be that they prefer not to block sweating, not fear of a link that has repeatedly failed to appear.

Cosmetic in the US — FDA primary category (12) personal cleanliness — because it addresses odour, which is an appearance-and-cleanliness function. A product that reduces perspiration is an OTC drug under the antiperspirant monograph.

An antibacterial claim would also move it: FDA's 2016 rule banned nineteen ingredients, including triclosan, from consumer antiseptic washes, and antibacterial claims elsewhere face the same scrutiny.

Aerosol versions carry the 21 CFR 740.11 warning statement for self-pressurised containers, and chlorofluorocarbon propellants, vinyl chloride in aerosols and zirconium-containing complexes in aerosols are three of the eleven entries FDA specifically prohibits.

In the EU, deodorants are cosmetics; aluminium chlorohydrate and related salts are assessed and restricted for antiperspirant use under Annex III, and named fragrance allergens must be declared individually above 0.001% in leave-on products.

Two to three years unopened; 12–24 months after opening. Sticks are anhydrous or low-water and very stable; creams in jars follow the ordinary tub caveat.

Aerosols fail by clogging. A stick that has crumbled, sweated oil or hardened has been heat-cycled — the usual cause is a gym bag or a car.

• 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 Underarm — Antiperspirant.
• Foot Powder and Spray — the same odour chemistry, on feet.
• Regulatory statements here follow the FDA and EU sources listed on Cosmetics, read 16 August 2026.