Baking soda in deodorant is the ingredient most often blamed when a natural deodorant stings, and the reason plenty of people give up on the whole category. The reputation is partly deserved and partly folklore. I run Lifelong, so weigh my view accordingly, and I'm not a dermatologist. Below I separate what the research establishes from what is inferred.

Quick answer: baking soda (sodium bicarbonate) is in deodorant because it is a mild alkali that reacts with the acidic compounds skin bacteria produce from sweat, which takes the edge off the smell. A minority of people react to it because it is more alkaline than skin. Many use it with no trouble at all.

What does baking soda do in deodorant?

Fresh sweat is largely odourless. Natsch and colleagues showed in the Journal of Biological Chemistry (2003) that an enzyme from armpit bacteria releases volatile acids such as 3-methyl-2-hexenoic acid from odourless precursors in apocrine sweat.

Because those smells are partly acidic, an alkali can neutralise some of them. That is the usual reasoning behind bicarbonate, and the chemistry is plausible, but I found no published trial testing it alone against placebo.

Why does baking soda irritate some people?

The plausible answer is pH. Two reviews put the normal skin surface at roughly pH 4 to 6: Proksch, Journal of Dermatology (2018), gives 4.1 to 5.8, and Panther and Jacob, Journal of Clinical Medicine (2015), give 4 to 6. Sodium bicarbonate is alkaline. A freshly made 0.1 molar solution measures about pH 8.3, and a 1% solution falls between 8.0 and 8.6, according to the PubChem record (citing the Merck Index and EU food-additive specifications).

Why would that gap matter? Panther and Jacob explain that the enzymes which build the skin's barrier lipids, acid sphingomyelinase and beta-glucocerebrosidase, work best at about pH 4.5 and 5.6. Enzymes that loosen the links between surface cells work faster as pH rises. Panther and Jacob also report that alkaline soap lifted hand-skin pH by about three units for around 90 minutes, and Proksch (2018) notes that irritant contact dermatitis and atopic dermatitis both involve a raised skin pH alongside a disturbed barrier.

What is established, what is inferred

• Established: skin is mildly acidic, bicarbonate solutions are alkaline, and raised skin pH goes hand in hand with barrier problems in conditions like eczema.
• Inferred: that daily bicarbonate on the underarm is enough to cause that disruption in healthy skin. A mechanism, not a measured result.
• Not found: a controlled patch-test study measuring how often bicarbonate deodorants irritate. I found only brand blogs and patents.

There is a further wrinkle. Underarm skin is already less acidic than the forearm. Lukacs and colleagues (Journal of the Society of Cosmetic Chemists, 1991) noted that skin-fold surface pH is markedly higher than the normal value of about 5.5. So the gap is smaller than the headline numbers suggest, though I can't say how much that matters.

Formulation matters too. A 2022 Cosmetic Ingredient Review assessment of related carbonate salts (International Journal of Toxicology) judged them safe as used when formulated to be non-irritating. Deodorant patents discuss irritation tied to coarse particles and high concentrations, though patents are claims, not trials.

Baking soda rash or just the adjustment period?

People confuse two things here. Switching from an antiperspirant can make you smell stronger for a while, because you sweat more and your armpit bacteria shift. In a small 2016 study in PeerJ (Urban and colleagues, 18 people), bacterial counts rose when participants stopped using antiperspirant. Nobody has measured how long the settling-in takes; the "two to four weeks" you see everywhere comes from brands, not trials. Our honest timeline for natural deodorant covers what people report.

A skin reaction is different. The NHS describes irritant contact dermatitis as itchy, dry, cracked or blistered skin, red on lighter skin and darker brown, purple or grey on darker skin, usually appearing within hours or days of contact.

• Adjustment: more smell or more sweat, but the skin itself feels normal.
• Reaction: stinging, burning, itching, redness or darkening, dryness or a rash, often worse after each application.

If it burns, that is the skin talking, not detox. See why natural deodorant burns underarms for other culprits.

Who is most likely to react to baking soda

No one has published a ranking, so this is reasoning from the pH evidence, not data:

• Eczema-prone skin. Panther and Jacob describe atopic skin as often neutral to alkaline, with a weaker barrier.
• Freshly shaved or broken skin. Plausible, and widely reported, but I found no study testing this for bicarbonate.
• High concentrations. Formulas where bicarbonate is near the top of the ingredient list.

What to use instead of baking soda

Alternatives do different jobs: some neutralise or bind odour, some fight bacteria, some only absorb damp.

Magnesium hydroxide. Patents and a recent review support it as an odour-neutralising active, but the review itself calls for randomised trials. It is also an alkali, so "gentler than baking soda" is a claim I couldn't verify.

Zinc oxide. This has the best human evidence of the group. A randomised, double-blind trial in Acta Dermato-Venereologica (2020) gave 30 volunteers zinc oxide on one armpit and placebo on the other for 13 days. It reduced odour-producing Corynebacterium and self-rated odour — the only active here with that level of human evidence behind it.

Zinc ricinoleate rests largely on patents and a molecular-modelling study: well used, not well tested. Triethyl citrate: the 1991 double-blind trial cited earlier found no significant odour reduction when it was added to a perfumed deodorant.

Arrowroot and cornstarch manage dampness, not smell. More in our guide to the best ingredients for sensitive underarms.

How to patch test a deodorant properly

There is no official home protocol, so this is a cautious routine, not a standard.

1. Use a small amount on a patch of clean, unbroken skin, such as the inner forearm.
2. Wait at least 48 hours; the NHS says contact dermatitis symptoms usually appear within hours or days.
3. If skin stays calm, try the underarm on skin that hasn't just been shaved.
4. Stop at the first stinging, redness or rash and rinse with warm water.

According to the NHS, a pharmacist can recommend emollients, and you should see a GP if symptoms are persistent, recurrent or severe. I can't offer medical advice beyond that.

Frequently Asked Questions

Is baking soda bad for your armpits?

Not for everyone. Many people use it for years without a problem. Bicarbonate is alkaline and skin is mildly acidic, which may irritate some people, especially with eczema-prone or freshly shaved skin. I found no controlled study of how often.

Why does my natural deodorant burn?

Burning can come from baking soda, but also from fragrance, essential oils, shaving irritation or broken skin. If it stings, stop using it and let the skin settle. Our burning guide helps narrow it down.

How do I know if I'm reacting to baking soda?

You can't be certain without testing. Stinging, redness or darkening, dryness and itching that worsen with each use point to a reaction. A stronger smell with comfortable skin is more likely the adjustment period. For certainty, see a GP or pharmacist.

Can baking soda deodorant cause a rash?

It can in some people. The NHS calls irritant contact dermatitis the more common type, where a substance directly damages the skin's outer layer. There is no good data on how often bicarbonate deodorant triggers it, so stop using it and seek advice if a rash appears.

Is magnesium hydroxide better than baking soda?

It is different, not proven better. Magnesium hydroxide is also alkaline, and the claim that it is gentler comes mainly from marketing. The published evidence for it as a deodorant is limited, so a patch test is still sensible.

How long does it take to adjust to natural deodorant?

Nobody has measured it properly. Brands often say two to four weeks, and a small 2016 study showed armpit bacteria shift after stopping antiperspirant. Skin pain or rash isn't part of it.

A note on Lifelong: our formula uses arrowroot and zinc oxide and contains no sodium bicarbonate. The Lifelong Luxe applicator is £49, Lifelong Vibes is £14.99, and refills are £8.99.