Why Does My Deodorant Burn?

If your deodorant burns, stings, or leaves your underarms red and raw, you're not imagining it and your skin isn't “too sensitive.” Roughly one in five people has a contact allergy to something, and the underarm is one of the easiest places on the body to provoke.¹

But the usual advice, “it's the baking soda,” is only half right, and only for half of you. There are two different versions of this problem. Which one you have depends on what's in the stick.

First: why the armpit

Underarm skin isn't like the skin on your arm. It has weaker barrier function, faster cell turnover, and a different lipid profile. It's warm, it's folded, it's occluded by your own arm all day, and it moves constantly.

Then most people shave it. Shaving strips away layers of stratum corneum along with the hair, which both irritates the skin directly and makes it absorb whatever you put on next far more readily.

That's about the worst-case setup for any ingredient with irritant potential, which is why the same formula that's fine on someone's arm can be miserable in their armpit.

Burning is a clue

This part is genuinely useful for working out what's happening to you. Dermatologists split these reactions into two types.¹

So if the sensation is burn or sting, especially right after you apply, you're most likely looking at an irritant, which is a concentration problem rather than an allergy. That's good news, because irritants are dose-dependent and therefore fixable. If it's a delayed, intensely itchy rash a day or two later, that's more likely a true allergy, and the culprit list looks different.

If you use a conventional stick or gel

If you use a natural stick

Here it's almost always about alkalinity, and baking soda is the usual source. Somewhere along the way, “baking soda free” turned into a marketing badge, a shorthand for gentle. It isn't one. It's shorthand for one specific ingredient is absent, which tells you almost nothing about whether the product will be comfortable on your skin.

We make one formula that contains baking soda and two that contain magnesium hydroxide, so this is a slightly awkward section for us to write. But the honest version is more useful than the marketing version.

What baking soda is actually doing

Bacteria in your armpit metabolize sweat into small acidic and sulfur-containing molecules. Those are what you smell. Sodium bicarbonate neutralizes the acidic ones on contact, and by pushing surface pH up, it makes the environment less workable for the bacteria producing them.

It's genuinely effective. That's not in dispute, and it's why it shows up in so many natural formulas: cheap, stable, easy to source, and it works on day one with no adjustment period. The trade-off is that it works by moving surface pH, and skin barrier function is pH-sensitive.⁶

The pH story, told properly

Here's the version you'll find on most natural deodorant blogs: your skin is pH 4.7, baking soda is pH 9, and that four-point gap destroys your acid mantle. Two things are wrong with it.

First, your armpit isn't pH 4.7. That figure comes from studies measuring the inner forearm.⁶ Axillary skin measures considerably higher: one controlled study found baseline underarm pH around 6.6 in men and 5.8–5.9 in women.^7,8 Intertriginous skin sits higher than the rest of you by nature. The gap is real, but smaller than advertised, and it varies person to person before you apply anything at all.

Second, pH is a property of a solution, not of a powder in a stick. The pH quoted for an ingredient is the pH of a saturated solution of it. What lands on your skin is a dispersion in a wax and oil base, at some specific percentage, that only becomes chemically active as your own sweat dissolves it.

Why the same pH number means two different things

That self-limiting behavior is exactly why magnesium hydroxide is the base used in antacids and sodium hydroxide isn't. Which gives you the counterintuitive result:

That's the real reason magnesium hydroxide is better tolerated. Not because it's “pH balanced,” it isn't. Because it delivers its alkalinity slowly and self-limits.

Which is why the percentage is the whole ballgame

Once you understand it as a delivery problem rather than an ingredient problem, the dose logic follows. A formula with a small amount of sodium bicarbonate can be perfectly comfortable. A formula with a large amount of magnesium hydroxide can still bother reactive skin, because a bigger reservoir means more total buffering capacity and a longer sustained pH shift. Two products can list the identical ingredient and behave completely differently.

This isn't a theory we invented. Contact dermatitis research runs into the same thing constantly: patch test positivity for propylene glycol varies widely across studies, in part because at higher test concentrations the substance starts irritating people who aren't allergic to it at all.^3,4 Concentration drives the reaction.

There's a stability wrinkle with bicarbonate too, which almost nobody mentions. In aqueous systems it slowly loses CO₂ and converts toward sodium carbonate, a considerably harsher skin irritant. A bicarbonate formula's pH is not necessarily the same at month twelve as it was at month one.

What to actually look for on a label

Ingredient lists run in descending order by weight down to 1%. That gives you a usable proxy.

If you've reacted to a natural deodorant before, the useful question isn't “does this have baking soda in it.” It's “how much of an alkaline active is in this, and how fast does it release.”

How to work out which one is yours

How we handle it

We took the position that the alkaline load should be a deliberate decision rather than whatever happens to make the odor testing look good.