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Nonenal & Body OdorWhy Does One Armpit Smell Worse Than the Other?
Your two armpits are two different habitats, and about half of people have measurably different bacteria on each side. What that does to the smell is the part most people get wrong.
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In this guide
- What is actually different between your two armpits
- The handedness story has one study pointing each way
- Can anyone else actually tell
- The trial design that assumes your sides are the same
- Nobody has looked for the ageing molecule in an armpit at all
- What the two popular fixes are actually worth
- What is worth doing tonight
- When one-sided is worth a doctor visit
- What a bar of soap can do here, ours included
- The bottom line
People who notice this usually notice it the same way: same shower, same deodorant, same shirt, and yet one side is fine and the other is not. It feels like it should be impossible, because both armpits belong to the same person and get treated identically. The suggestion box agrees that it is common — type the first few words and you get versions asking about the left side, the right side, the sudden onset, and whether it happens even after washing.
It is common, and there is a real answer underneath it. But it splits into two questions that are usually run together, and separating them is the whole point of this article. The first is whether your two armpits are actually different. The second is whether anyone else can tell. The first is measured. The second mostly is not.
The short version
- Your two armpits are two separate habitats, and in roughly half of people sampled they carry measurably different bacterial communities. Since those bacteria are what make the smell, different bacteria can mean a different smell.
- In the one experiment that asked other people to judge, left and right odour samples were rated the same for right-handed donors.
- The handedness explanation that gets passed around has one small study pointing each way, and neither is decisive.
- The ageing molecule this site is mostly about, 2-nonenal, has never been measured in an armpit at all.
- A smell on one side that is new, rather than long-standing, is the version worth taking to a doctor.
What is actually different between your two armpits
The strongest piece of evidence here is a 2013 study that typed the underarm bacteria of 53 healthy people, using both gel electrophoresis fingerprinting and 16S rRNA gene sequencing. It found two dominant community types, one built around Staphylococcus species and one around Corynebacterium species, and it found that the make-up was unique to each person. The line that matters for this question is blunt: “Left-right asymmetry occurred in about half of the human population” (Callewaert et al., 2013).
So the observation is not imagination. One side can be running a different microbial community from the other, in something like half of people.
That matters because of where underarm smell comes from, which is not sweat. Apocrine sweat is close to odourless when it arrives, and the smell appears when particular skin bacteria take up an odourless precursor and release a volatile sulphur compound from it (Minhas et al., 2018). The bacteria are the step that makes the molecule. Change the bacteria and you have changed the chemistry, not just the quantity.
“one armpit smells bad, while the other doesn't”
The same paper carries a detail worth knowing before you attack one armpit with everything in the cupboard: deodorant use had a proportional linear influence on the species diversity of the underarm microbiome. Whatever you have been doing to one side is part of the system you are trying to explain.
The handedness story has one study pointing each way
The folk explanation is that you use one arm more, so that side sweats more, so that side smells more. It is a reasonable guess, and the literature is genuinely split on it.
A 2011 profiling study took skin scrub samples from both armpits of 10 healthy men and looked at active bacteria rather than merely present ones, by profiling rRNA alongside DNA. Its headline result points the opposite way to the folk story: the fingerprints of a person’s left and right armpits were “pronounced[ly] similar”, the big differences being between people rather than between sides. But one group broke the pattern. Bacteria related to Peptoniphilus were significantly more active in right armpits, and the authors suggested this “might have been” related to the handedness of their volunteers (Egert et al., 2011). The authors were employees of Henkel, a consumer goods company, which is worth knowing without being a reason to discount the work.
Ten men is ten men. Set that beside 53 people finding asymmetry in half the population and you do not have a contradiction so much as two small studies measuring different things — community membership in one, community activity in the other.
Can anyone else actually tell
This is the question the online discussion skips, and it is the one that has been tested properly. In 2009, a group at Liverpool recruited 38 men as odour donors and 49 women as raters. The donors wore cotton pads under each arm for 24 hours, and the left and right samples were presented to raters as independent stimuli and scored for intensity, attractiveness and masculinity — nobody was told which arm a pad came from, or that sides were the point.
The result was mostly a null. No side-related difference emerged in right-handed donors at all. The only effect appeared in left-handers, whose dominant-side odour was rated more intense and more masculine — and even that was limited to raters not using hormonal contraception and estimated to be in the fertile phase of their cycle. The authors’ conclusion is that for a population-representative sample, left and right samples “can [be considered] perceptually equivalent stimuli” (Ferdenzi et al., 2009).
Put the two findings together and you get the honest shape of this subject. The bacteria differ in about half of people; the perceived smell, for most donors and most raters, does not. Both can be true, because a difference in community composition need not be large enough to clear the threshold of somebody else’s nose.
The trial design that assumes your sides are the same
There is a neat reflexive check on all this. One standard way to test an underarm product is to treat one armpit and leave the other as the control, with neither the participant nor the assessor knowing which is which. A 2020 Danish randomised, double-blind trial of zinc oxide did exactly that with 30 volunteers (Ågren et al., 2020), and we have leaned on it elsewhere, in what the evidence for acid-based deodorising shows and in what a fungal explanation can account for.
That design only makes sense if a person’s two armpits are close enough to interchangeable to serve as each other’s control. Researchers use it constantly, which makes it quietly the best evidence that the typical left-right difference is small — and the 2009 perception study is more or less the paper that licenses it.
Nobody has looked for the ageing molecule in an armpit at all
This site exists mostly because of a different molecule: 2-nonenal, the greasy, grassy aldehyde that the original 2001 work found in the body odour of older people and not younger ones. It comes from skin surface lipids oxidising, which is a separate pathway from the bacterial one described above.
So: is it worse on one side? Nobody knows. A title-and-abstract search pairing 2-nonenal with axilla, axillary, armpit and underarm — four different words for the same place, so this is not a vocabulary problem — returns zero records. The molecule the entire anti-nonenal product category is built around has never been measured in an armpit, never mind compared between a person’s two armpits.
That is worth saying plainly, because marketing in this category, ours included, invites you to think of the underarm as the home of the ageing smell. The original measurements came from skin surface lipids and whole-body odour collection, and the greasy smell people describe is usually reported from the neck, scalp and upper back rather than the pits.
What the two popular fixes are actually worth
The thread that prompted this article offers two remedies, and both deserve a straight answer.
The first is transferring bacteria from the good side to the bad side on a cotton pad. This is a real research idea, not an internet invention: a 2017 viewpoint article makes the case for replacing an odour-causing underarm microbiome with a non-odour-causing one, by transplantation or by applying selected bacteria, and notes that the relevant organisms live not just on the surface but in the sweat glands, pores and hair follicles (Callewaert et al., 2017). The same lead author later reviewed the field (Callewaert et al., 2021). Read the verbs, though: the 2017 title begins Towards, and its abstract says the article envisions a bacterial treatment. Moving bacteria between your own armpits is very unlikely to hurt you, and it has no published trial behind it for this problem.
The second is daily lemon juice. Here the honest answer has a safety edge on it.
Citrus on skin, then sun, is a known reaction. Lime and lemon juice contain furocoumarins. On skin and then exposed to ultraviolet light, these produce phytophotodermatitis — described in a 2024 review as “a non-immunologic skin reaction triggered by certain plants followed by exposure to ultraviolet radiation”, causing redness, fluid blisters, swelling and lasting discolouration (Grosu et al., 2024). Separately, the FDA reports that after four weeks of alpha hydroxy acid use, sensitivity to UV-induced skin reddening rose by 18 percent and sensitivity to UV-induced cellular damage roughly doubled, reversing about a week after stopping (FDA, Alpha Hydroxy Acids).
And for the odour claim itself: pairing body odour with lemon juice, citrus or citric acid in a title-and-abstract search returns nothing at all.
What is worth doing tonight
Nothing below is a treatment, and none of it will act in a day.
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Treat the two sides as two problems, not one. If one side responds to a change and the other does not, that is information. It is also the single thing most people in these threads never try, because the instinct is to escalate on both sides at once.
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Change one variable at a time, and give it two weeks. Bacterial communities are described in the literature as relatively stable over time, which cuts both ways: they do not flip overnight because you switched soap on Tuesday.
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Wash both sides properly and dry them properly, and keep citrus off your skin before going outside. The NHS self-care advice sits underneath everything else: wash the armpits, groin and feet at least twice a day with soap and dry thoroughly, change and wash clothes regularly, and favour natural fabrics (NHS).
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Check price →We own TanniBar and earn from sales. 30-day refund, no return needed.When one-sided is worth a doctor visit
The version of this question that deserves medical attention is the new one, not the long-standing one. If a side has smelled different for years and nothing else has changed, that is the ordinary finding described above. If it started recently, the picture is different.
See a GP if your body odour has not improved after treating it yourself and you notice a change in how it usually smells, you suddenly begin to sweat much more than usual, or it is affecting your self-esteem. The NHS also notes that conditions such as diabetes, kidney disease and liver disease, and some medicines including antidepressants, can make body odour worse (NHS: Body odour). A new, clearly one-sided change in how much you sweat is worth mentioning too, because sweating on each side is under nerve control and a clinician will want to know.
“Just my right armpit smells funny”
This question also arrives disproportionately from people in their forties and fifties, which is why the perimenopause version of it is a topic of its own. A hormonal change affects both sides at once; what it can do is make a pre-existing difference newly noticeable.
What a bar of soap can do here, ours included
This is the part where we are supposed to tell you that the right soap fixes it. It does not, and the reason is in the sources above.
A rinse-off bar works on the surface of the skin. The bacteria that make underarm smell are described in the research as living in the sweat glands, sweat pores and hair follicles, not only on the epidermis — which is why the researchers proposing to change them talk about transplantation rather than washing. Washing reduces the load for a while; it does not re-colonise anything. And the molecule our own product is built around has, as the search above shows, never been measured in an armpit at all.
So for this specific question, a persimmon tannin bar is not the answer, ours included. TanniBar is a fragrance-free true soap that gets rinsed off: it can remove added scent as a variable, and it cannot change which bacteria live in your follicles. If you are weighing it against the alternatives anyway, our persimmon soap comparison is the place to do that, with the usual caveat that we own the bar sitting at the top of it.
The bottom line
Your two armpits really are two different places, and in about half of people they carry measurably different bacteria. What does not follow — and what the one published perception experiment found against — is that other people can usually tell. If one side has always been worse, that is real and common, and scrubbing it harder will not change which organisms live in it. If one side has recently become worse, especially alongside a change in how much you sweat, take it to a GP rather than to a shelf.
And the thing nobody can tell you, because nobody has run the experiment: whether the ageing smell this whole category is built on is worse on one side than the other. The search returns zero. We would rather say so than guess.
Frequently asked questions
Why does one armpit smell worse than the other?
The best-supported answer is that your two armpits are not carrying the same bacteria. In a study of 53 healthy people whose underarm microbiota were typed by gel electrophoresis and 16S sequencing, the authors reported that left-right asymmetry occurred in about half of the population sampled. Underarm smell is made by bacteria acting on otherwise odourless compounds in sweat, so two sides with different bacterial communities can plausibly produce different amounts and different kinds of smell. What has not been shown is that the difference is usually large enough for another person to notice.
Is it because of which hand I use?
The evidence points both ways and all of it is small. A 2011 profiling study of 10 healthy men found the overall bacterial fingerprints of a person's left and right armpits to be strongly similar, with one exception: bacteria related to Peptoniphilus were significantly more active in right armpits, which the authors suggested might relate to the handedness of their volunteers. A separate perception experiment found a side difference only in left-handed donors, and only when a restricted subgroup of raters judged the samples. Neither study is big enough to settle it, and they do not agree.
Can other people actually tell that one side smells worse?
In the one published experiment that asked, usually not. Thirty-eight men wore cotton pads under each arm for 24 hours and 49 women rated the left and right samples as separate stimuli for intensity, attractiveness and masculinity. No side-related difference emerged in right-handed donors. The only effect was in left-handers, whose dominant-side odour was rated more intense and more masculine, and only by raters who were not using hormonal contraception and were estimated to be in the fertile phase of their cycle. The authors concluded that for a population-representative sample, left and right samples can be treated as perceptually equivalent.
Should I worry if one armpit suddenly starts to smell different?
A sudden change is the version of this question worth taking to a doctor, rather than the long-standing version. The NHS advises seeing a GP, not urgently, if body odour has not improved after treating it yourself and you notice a change in how it usually smells, you suddenly begin to sweat much more than usual, or it is affecting your self-esteem. The same page notes that conditions such as diabetes, kidney disease and liver disease, and medicines such as antidepressants, can make body odour worse. One-sided sweating that is new is also worth mentioning, because the amount you sweat on each side is under nerve control.
Does rubbing the good armpit onto the bad one work?
It is a real research idea, but it is not an established treatment. A 2017 viewpoint article in Experimental Dermatology set out the case for replacing an odour-causing underarm microbiome with a non-odour-causing one, through bacterial transplantation or direct application of selected bacteria. The word the authors use for it is envisions. A 2021 review by the same lead author surveys skin microbiome transplantation and manipulation as an emerging field. Transferring bacteria between your own armpits with a cotton pad is unlikely to harm you, but nobody has published a trial showing it fixes a one-sided smell.
What about putting lemon juice on the armpit that smells?
There is no published evidence for it, and there is published evidence against doing it before sun exposure. A title-and-abstract search pairing body odour with lemon juice, citrus or citric acid returns no records at all. Meanwhile lime and lemon juice contain furocoumarins, which react with ultraviolet light to produce phytophotodermatitis, a non-immunologic burn-like reaction with redness, blistering and lasting discolouration. The FDA also reports that after four weeks of alpha hydroxy acid use, sensitivity to UV-induced reddening rose by 18 percent and sensitivity to UV-induced cellular damage roughly doubled, reversing about a week after stopping.
Is the ageing smell, nonenal, worse on one side?
Nobody knows, because nobody has looked. Pairing 2-nonenal with axilla, axillary, armpit or underarm in a title-and-abstract search returns zero records. The molecule the whole anti-nonenal product category is built around has never been measured in an armpit at all, let alone compared between a person's two armpits. The original work that found it measured skin surface lipids and whole-body odour samples, not underarm swabs, and 2-nonenal comes from oxidising skin oil rather than from the bacterial pathway that produces classic underarm smell.
Sources
- Callewaert C., Kerckhof F.M., Granitsiotis M.S., Van Gele M., Van de Wiele T., Boon N. (2013). Characterization of Staphylococcus and Corynebacterium clusters in the human axillary region. PLoS One 8(8):e70538. (53 healthy subjects, axillary microbiota typed by denaturing gradient gel electrophoresis and 16S rRNA gene sequencing. Two clusters were characterised, dominated by Staphylococcus and by Corynebacterium species. "Females predominantly clustered within the Staphylococcus cluster (87%, n=17), whereas males clustered more in the Corynebacterium cluster (39%, n=36)." "The axillary microbiota was unique to each individual. Left-right asymmetry occurred in about half of the human population." "The deodorant usage had a proportional linear influence on the species diversity of the axillary microbiome.")
- Egert M., Schmidt I., Höhne H.M., Lachnit T., Schmitz R.A., Breves R. (2011). rRNA-based profiling of bacteria in the axilla of healthy males suggests right-left asymmetry in bacterial activity. FEMS Microbiol Ecol 77(1):146-53. (DNA and RNA isolated from skin scrub samples taken from both armpits of 10 preconditioned healthy males, profiled by terminal-restriction fragment length polymorphism with cloning and sequencing. "The fingerprint profiles indicated pronounced similarities between the armpit microbiota in the right and the left axillae of an individual test person, but larger differences between the axilla microbiota of different individuals." "Surprisingly, the relative abundance of sequences affiliated with Peptoniphilus sp. was by far and with statistical significance the highest in the rRNA samples of the right armpits." The authors suggest this "might have been" related to "the handedness of the test persons". The authors were affiliated with Henkel AG & Co. KGaA, a consumer goods company.)
- Ferdenzi C., Schaal B., Roberts S.C. (2009). Human axillary odor: are there side-related perceptual differences? Chem Senses 34(7):565-73. (38 male odour donors and 49 female raters, aged 19-35. Fresh odour samples were cotton pads worn underarm for 24 hours, with left and right samples presented as independent stimuli and rated for attractiveness, intensity and masculinity. "A side-related difference emerged in left-handers only (no difference in right-handers): The odor from the axilla corresponding to the dominant side (left) was rated more masculine and more intense than the other side (right). This effect was limited to the ratings of a restricted group of females, that is, those who did not take hormone-based contraception and were estimated to be in the fertile phase of their menstrual cycle." The authors conclude that "future studies using axillary odor samples can consider left and right samples as perceptually equivalent stimuli when the participant samples are representative of the general population".)
- Minhas G.S., Bawdon D., Herman R., Rudden M., Stone A.P., et al. (2018). Structural basis of malodour precursor transport in the human axilla. Elife 7:e34995. (Underarm malodour is produced when specific skin bacteria, principally Staphylococcus hominis, take up an odourless precursor secreted in apocrine sweat and release the volatile thioalcohol 3-methyl-3-sulfanylhexan-1-ol. This is the bacterial pathway behind classic underarm smell, and it is distinct from the oxidative pathway that produces 2-nonenal from skin lipids.)
- Callewaert C., Lambert J., Van de Wiele T. (2017). Towards a bacterial treatment for armpit malodour. Exp Dermatol 26(5):388-391. (A viewpoint article, not a trial. "Not only the bacteria on the epidermis, but also and especially those living in the sweat glands, sweat pores and hair follicles play a pivotal role in malodour development." "To treat underarm malodour, this viewpoint article envisions a bacterial treatment. Replacing the autochthonous malodour-causing microbiome with a non-odour-causing microbiome, through an armpit bacterial transplantation or direct application of probiotics/non-odour-causing bacteria, could resolve the condition." The verb is envisions; the title begins with Towards.)
- Callewaert C., Knödlseder N., Karoglan A., Güell M., Paetzold B. (2021). Skin microbiome transplantation and manipulation: Current state of the art. Comput Struct Biotechnol J 19:624-631. (A review of skin microbiome manipulation strategies spanning transplantation, bacteriotherapy, prebiotics, probiotics and postbiotics, summarising the scientific data and clinical trials undertaken for common skin disorders. Presented by its authors as a novel platform for therapy rather than as established practice.)
- Ågren M.S., Gottrup F., Jorgensen L.N., et al. (2020). Zinc Oxide Inhibits Axillary Colonization by Members of the Genus Corynebacterium and Attenuates Self-perceived Malodour: A Randomized, Double-blind, Placebo-controlled Trial. Acta Derm Venereol 100(8):adv00145. (30 healthy volunteers, zinc oxide applied to one armpit and placebo to the other for 13 days, double-blind. Odour-producing Corynebacterium species and Staphylococcus hominis fell on the treated side and self-judged malodour fell with them, while skin surface pH rose rather than fell. Cited here for its split-body design: the two armpits are routinely used as treatment and control for each other.)
- Haze S., Gozu Y., Nakamura S., Kohno Y., Sawano K., Ohta H., Yamazaki K. (2001). 2-Nonenal newly found in human body odor tends to increase with aging. J Invest Dermatol 116(4):520-4. (2-Nonenal, "an unsaturated aldehyde with an unpleasant greasy and grassy odor, was detected only in older subjects (40 y or older)". Omega-7 unsaturated fatty acids and lipid peroxides in skin surface lipids both increased with age and correlated with the amount of 2-nonenal in body odour; the molecule was generated only when omega-7 unsaturated fatty acids were oxidatively degraded. The authors were affiliated with Shiseido, a cosmetics company.)
- Grosu E., Dumitrescu A., Jîjie A.R., Manea H.C., Moacă E.A., et al. (2024). New Insights Concerning Phytophotodermatitis Induced by Phototoxic Plants. Life (Basel) 14(9):1019. (A review of phytophotodermatitis, "a non-immunologic skin reaction triggered by certain plants followed by exposure to ultraviolet radiation emitted by sunlight". Plant-derived photosensitising compounds, principally furanocoumarins and psoralens, interact with ultraviolet light "leading to skin damage (e.g., erythema, fluid blisters, edema, and hyperpigmentation)". Rutaceae is named among the most common phototoxic plant families, and "limes containing furocoumarin have been linked to lime-induced photodermatitis".)
- PubMed title-and-abstract searches on sidedness and underarm odour, run on 30 October 2026. ("2-nonenal"[TIAB] AND (underarm*[TIAB] OR armpit*[TIAB] OR axillary[TIAB] OR axilla[TIAB]) returns 0 — four separate words for the same place, so this is not a vocabulary artefact. ("body odor"[TIAB] OR "body odour"[TIAB]) AND ("lemon juice"[TIAB] OR citrus[TIAB] OR "citric acid"[TIAB]) returns 0. bromhidrosis[TIAB] AND (unilateral[TIAB] OR asymmetr*[TIAB]) returns 1, and it is a survey of coryneform skin infections rather than a study of one-sided smell. ("body odor"[TIAB] OR "body odour"[TIAB] OR "axillary odor"[TIAB]) AND (handedness[TIAB] OR "dominant hand"[TIAB] OR lateral*[TIAB]) returns 7, walked in full: a car-cabin air quality study, an infant brain imaging study, a rodent predator-odour study, a binocular rivalry study, a schizophrenia olfaction study, and the Egert and Ferdenzi papers above. Only the last two are about the two sides of a human armpit.)
- FDA: Alpha Hydroxy Acids (accessed 30 October 2026). On the UV study: after four weeks of AHA use, sensitivity to UV-induced skin reddening "increased by 18 percent", and sensitivity to UV-induced cellular damage roughly doubled on average with "considerable differences among individuals"; one week after stopping, no significant differences in UV sensitivity remained. The Cosmetic Ingredient Review panel concluded such products are safe when "the AHA concentration is 10 percent or less" and "the final product has a pH of 3.5 or greater". "FDA received a total of 114 adverse dermatologic experience reports" for AHA-containing skin care products between 1992 and February 2004.
- NHS: Body odour (BO) (accessed 30 October 2026). Advises seeing a GP, though not urgently, if body odour has not got better after treating it yourself and "it's affecting your self-esteem", "you notice a change in how it usually smells", or "you suddenly begin to sweat much more than usual". Names "having a condition like diabetes, kidney disease or liver disease" and "certain types of medicine, such as antidepressants" among the things that can make body odour worse. Self-care advice includes washing the armpits, groin and feet at least twice a day with soap and drying thoroughly, using antiperspirants and deodorants, changing and washing clothes regularly, and wearing natural fabrics.
- r/bodyodor, "The reason one armpit smells and the other one doesn't", posted 13 October 2026. (A post describing fifteen years of research on underarm odour, which attributes side differences to the microbiome rather than to handedness or hormones and suggests transferring bacteria from the non-smelly side and applying lemon juice daily for two weeks. Cited as evidence of what readers are being told, not as a source for either claim.)
- r/Perimenopause, "Armpits be stanky", posted 21 October 2026. (A thread in which readers describe underarm odour changes during perimenopause, including one reader reporting that only one side is affected.)
- r/Perimenopause, "Armpits", posted 17 October 2026. (A 98-entry thread on underarm odour in which multiple readers independently report that one armpit, most often the left, smells worse than the other. Read for this article as evidence of how common the observation is; the explanations offered in the thread are readers' own speculation and are not reproduced here.)
- TanniBar product page (accessed 30 October 2026). States "100% fragrance-free", "Triple-milled to last a month of daily showers", and an ingredient list of persimmon tannin, green tea leaf extract, vegetable glycerin, shea butter, saponified coconut and palm oil, vitamin E and water. Also states "It is a cosmetic product, not a medicine, and is not a substitute for medical advice" and "If body odour changes suddenly or is accompanied by other symptoms, see a healthcare professional." The Soap Digest and TanniBar share an owner.
