In order to understand why, it's important to understand why your armpits smell in the first place. Contrary to popular belief, it's actually not solely because of your sweat. Instead, it's because of how your sweat interacts with the bacteria living on the surface of your skin, which can release an odor. Pure deodorants work to mask this odor with fragrance and alcohol, which acidifies the sweat and makes it a less hospitable place for bacteria to live. Antiperspirants—a category that some, but not all, deodorants fall into—use an active ingredient (usually aluminum) to plug up the sweat glands. This stops sweat from making it up to the surface in the first place so that bacteria can't feed on it. When either type of product is working properly, the result is going to be a reduction in body odor.
If that's stopped happening, though, it may not technically be the deodorant's fault—it could be due to some underlying cause in your body. "Perceived differences in smell may have to do with how much the person is actually sweating," says board-certified dermatologist and founder of LM Medical Morgan Rabach, MD. Things like stress, hormonal changes, new medications, and different lifestyle factors (like harder workouts or shifts in the weather) can all impact how you sweat, and your go-to deodorant may not be able to stand up to the new situation. "There also can be changes in the natural bacterial flora that live on the skin, [as in] increased amounts, that lead to more bacteria interacting with the sweat and making the odor," she adds.
Another possibility is that there's been too much product buildup under your armpit, which can lead to a lack of effectiveness. "Deodorants coat the underarm skin, layering a film on the armpit, and the residue stays on even after normal bathing," says board-certified dermatologist and founder of Surface Deep Anti-Odorant Alicia Zalka, MD. If you're using an anti-perspirant, this could cause your skin to lose its response to the active ingredient (usually aluminum) that's traditionally been able to keep sweat at bay. "When that happens, the sweat gland and its sweat duct will return to its normal function of delivering sweat to the surface of the skin," says Dr. Zalka.
In order to fix the situation, she suggests pressing the "reset" button on your deodorant routine. Take a break from using product on your armpits for three or four days to allow them to return to their natural state, then test your go-to product again. If it still doesn't work, it may be time to try something new.
We asked a professional armpit sniffer (yup—that's a thing) to answer all of your most pressing D.O. and B.O. questions, plus, why you may want to consider swiping your underarms with an exfoliating acid.
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