Your armpits smell because bacteria living there digest the odorless secretions of your apocrine sweat glands and release pungent sulfur compounds and fatty acids. A small group of microbes does most of this work, chiefly Staphylococcus hominis and some Corynebacterium species, so the smell depends on your armpit microbiome as much as on how much you sweat.[1][2][3]
That also explains the frustrating parts: why odor can come back hours after a shower, why it smells like onions one day and sour the next, and why some people barely smell at all. This guide walks through the bacteria involved, what each smell points to, and which fixes have evidence behind them.
What causes BO, and why does body odor smell so bad?
Body odor is made on the skin, not in the gland: fresh armpit secretions themselves are odorless.[2] The armpit hosts a large, permanent population of microbes that live on secretions from three kinds of glands: eccrine (watery sweat), apocrine and sebaceous (oil).[3] The armpit's apocrine glands do not begin to function until puberty, and their secretory cells carry receptors for sex hormones.[4] The key odor precursors are believed to come from these glands.[3]
Those precursors arrive with the odorant chemically locked to a carrier. In fresh, odorless armpit secretions, the acids behind classic body odor are bound to the amino acid glutamine, and armpit Corynebacterium strains cut them free with a dedicated enzyme.[2] Sulfur odorants arrive bound to a two-amino-acid tag (cysteinyl-glycine), which certain staphylococci pull inside and cleave.[5]
The odor molecules fall into three main classes:[3]
- Thioalcohols, sulfur compounds that are the most pungent part of body odor[1]
- Short- and medium-chain volatile fatty acids, including 3-hydroxy-3-methylhexanoic acid, which smells cheesy and rancid[6]
- 16-androstene steroids
Which bacteria make your armpits smell so bad?
A handful of species do most of the work, and close relatives differ sharply. In a library of skin isolates, Staphylococcus hominis, S. haemolyticus and S. lugdunensis converted the sulfur precursor efficiently, while Staphylococcus epidermidis and Corynebacterium tuberculostearicum, two of the most common armpit residents, produced little.[5]
The difference comes down to one enzyme. A 2020 study found a cysteine-thiol lyase only in a branch of odor-forming staphylococci, and moving that gene alone into a non-odorous staphylococcus made it produce thioalcohols.[1] The enzyme appears to have entered that branch about 60 million years ago, which suggests body odor is an ancient trait.[1]
Real armpits fit the lab work:
- Among 24 volunteers rated by trained assessors after an indoor cycling session, the malodorous group carried relatively more S. hominis, and their S. hominis isolates had higher sulfur-releasing (C-S lyase) activity.[7]
- In 11 people who did not use antiperspirant, 96% of armpit bacterial sequences belonged to Staphylococcus, Corynebacterium and Propionibacterium. Corynebacterium rose with odor, Propionibacterium fell with it, and specific S. hominis and Anaerococcus groups tracked particular odor notes.[8]
Corynebacterium is a large genus, and not every member is an odor maker; Corynebacterium accolens, for example, is covered on Biome Guide for its role in the nose.
| Microbe | Role in armpit odor | Evidence |
|---|---|---|
| Staphylococcus hominis | Efficient thioalcohol producer; more abundant and more active in people with stronger odor | Lab isolates and a 24-person study[5][7] |
| S. haemolyticus, S. lugdunensis | Efficient thioalcohol producers | Lab isolates[5] |
| Corynebacterium (for example C. striatum) | Releases odorous acids from glutamine-bound precursors; abundance tracks odor | Enzyme study and sequencing[2][8] |
| Staphylococcus epidermidis | Very common resident; low thioalcohol producer | Lab isolates[5] |
| Anaerococcus, Peptoniphilus | More common in men; one Anaerococcus group tracked odor | Sequencing[8] |
| Propionibacterium | Higher abundance went with less odor | Sequencing[8] |
| Micrococci | Grew almost only on polyester shirts, which smelled worse | Textile study[9] |
Why does my sweat smell like onions, vinegar or cat pee?
The particular note hints at which chemistry dominates. Women's secretions can release relatively more of the onion-like thioalcohol, while men's carry about three times more acid precursor relative to sulfur precursor.[6] In the same study, the sweat samples with the strongest sulfur note were rated the most intense and most unpleasant.[6]
| What you notice | Likely compound | Where it comes from |
|---|---|---|
| Onion, tropical fruit | 3-methyl-3-sulfanylhexan-1-ol, a thioalcohol | Released from a cysteinyl-glycine precursor by S. hominis and related staphylococci[6][5] |
| Cheesy, rancid | 3-hydroxy-3-methylhexanoic acid and 3-methyl-2-hexenoic acid | Cut from glutamine-bound precursors by Corynebacterium[2][6] |
| Sour, vinegary, acid-spicy | Short-chain fatty acids | Formed in people of every ABCC11 genotype; the acid-spicy note rose through the day[10][8] |
| Sulfury, cat urine | Sulfur notes; no single compound named | A trained-panel descriptor that rose from morning to afternoon[8] |
| Greasy, grassy (age 40 and up) | 2-nonenal | Oxidation of omega-7 fatty acids in skin-surface oil[11] |
| Rotting fish | Trimethylamine | Fish-odor syndrome (trimethylaminuria), a disorder of liver metabolism[12] |
A sour or vinegar-like smell, then, fits the short-chain fatty acid side of the chemistry, which even people with the low-odor gene variant produce.[10] A sharp onion smell points more to the sulfur side and the staphylococci that make it.[6][5]
Why do my armpits smell even after a shower?
Washing does not reach every odor-making microbe, and the conditions that rebuild odor come back as the day goes on.
- Bacteria live below the surface. Microbes in the sweat glands, pores and hair follicles, not only on the skin, play a pivotal role in odor.[13]
- Odor builds through the day. In one study, overall sweat odor and the sulfury and acid-spicy notes generally rose from morning to afternoon.[8]
- Hair holds on to it. In a randomized split-body trial in 30 men, soap washing after shaving cut odor by 57.3% and after waxing by 75.3%, while clipping with scissors (27.2%) was no better than soap alone.[14]
- Polyester holds on to it too. T-shirts worn in a spinning class and kept for 28 hours smelled significantly less pleasant and more intense in polyester than in cotton, and micrococci (the group that includes Micrococcus luteus) grew almost only on the synthetic shirts.[9]
- Stress adds fuel. A standardized stress test produced large amounts of sweat and strong armpit odor in 40 teenagers, with higher odor scores in boys.[15]
- Recently stopped products. When people stopped deodorant and antiperspirant, culturable bacteria climbed toward the levels seen in habitual non-users.[16]
Why do some people barely smell at all?
Mostly genetics. A single variant (rs17822931) in the ABCC11 gene determines both earwax type and armpit odor.[17] People with two copies of the A version secrete fewer apocrine odor precursors and have significantly less armpit odor, although trained assessors still detected some.[10] The A version is more common in East Asians,[17] and this mutation can explain why a large share of people in the Far East lack typical body odor formation.[18]
In a UK cohort of about 17,000 people, carriers of two A copies were almost five times overrepresented among those who never or rarely used deodorant. Still, 77.8% of white Europeans with the low-odor genotype used it anyway.[17]
Other factors shift odor less dramatically:
- Sex: men's armpit odor was rated more intense, and their armpits carried more bacteria.[8]
- Red meat: in a two-week crossover in 17 men, odor on a meat-free diet was rated more attractive, more pleasant and less intense.[19]
- Garlic: despite its reputation, garlic did not worsen armpit odor in three studies of 42 men. At higher doses, odor was rated more pleasant and less intense.[20]
- Age: 2-nonenal appeared only in people aged 40 or older.[11]
What kills armpit odor fast?
The fastest results come from cutting the bacterial load and the trapped moisture together.
How to remove underarm smell at home
Here is what the evidence supports:
| Step | What the research shows |
|---|---|
| Wash daily with soap, then shave | A single shave made soap washing more effective for three days in men[14] |
| Use an antiperspirant | Non-users had about 50 times more armpit bacteria and stronger odor than antiperspirant users[8]; applying antiperspirant sharply cut bacterial density[16] |
| Wear cotton for workouts | Polyester shirts smelled more intense and less pleasant after exercise[9] |
| Eat less red meat | Odor was rated less intense after two meat-free weeks[19] |
| Treat heavy sweating | Prescription antiperspirants, medicated wipes, botulinum toxin and other options are available[21] |
Some popular fixes have little behind them. For chlorophyll tablets, the only study we read was an uncontrolled 1980 report in 62 nursing-home patients, and its own lab tests did not confirm the antibacterial effect claimed for chlorophyllin.[22]
Deodorant vs antiperspirant: which works better?
An antiperspirant temporarily blocks sweat glands, while a deodorant reduces odor.[21] Antiperspirant may do double duty: in one sequencing study, antiperspirant users had weaker sweat odor and about 50 times fewer armpit bacteria than people who did not use antiperspirant.[8] For heavy sweating, dermatologists advise applying antiperspirant to dry skin at bedtime and letting it dry before bed.[21]
The two also shape the microbiome differently. In a month-long Belgian study, antiperspirant use raised the share of Actinobacteria, a shift the authors described as unfavorable for body odor.[23] People who habitually used no product had armpits dominated by Corynebacterium.[16]
Is aluminum in antiperspirant bad for you? Aluminum is the active ingredient in many antiperspirants, and the American Academy of Dermatology states that research has not found aluminum antiperspirants to cause Alzheimer's disease or breast cancer.[21]
Does shaving your armpits reduce odor?
Yes, at least for a day or so. In the split-body trial, shaving plus washing cut odor by 57.3%, and the benefit persisted for about 24 hours.[14] The trial included only men, so results in women are less certain.
Medical options when sweat is the driver
Over-the-counter antiperspirants help only mild hyperhidrosis (excessive sweating), and many people need a prescription product.[21] Other options dermatologists use include glycopyrronium cloths and sofpironium gel for the underarms, and botulinum toxin injections, whose effect in the underarms tends to last 3 to 10 months.[21] Microwave thermolysis destroys sweat glands with heat and, according to the AAD, studies show it can also get rid of underarm odor and hair.[21]
Can you change your armpit microbiome?
Partly, and it already changes with your habits. Each person's armpit community is unique and stays stable on a steady routine, but switching between daily product use and none shifts it distinctly.[23] Long-term product users who stopped for two or more days ended up dominated by Staphylococcaceae, while habitual non-users carried mostly Corynebacterium.[16]
Researchers have proposed going further: armpit bacterial transplants, applying non-odorous bacteria as probiotics, or steering the community with prebiotics or plant extracts.[13] These ideas come from a viewpoint article, not from large trials. The enzyme work offers a sharper target, since a single staphylococcal enzyme is both necessary and sufficient for thioalcohol production.[1]
When should you see a doctor about armpit odor?
Most armpit odor is normal biology. Body odor reflects metabolism, though, and infections or metabolic diseases often change it.[24] See a clinician if you notice:
- A sudden change in your usual smell that you cannot explain[24]
- A fishy smell that washing does not remove. In fish-odor syndrome, the liver enzyme FMO3 fails to convert dietary trimethylamine into its odorless N-oxide, TMAO; the inherited form is persistent, and liver dysfunction can also cause it[12]
- Sweating that soaks your clothes or interferes with daily life, which may be treatable hyperhidrosis[21]
- A red, itchy or scaly rash along with the smell, which needs a different approach; see our guides to armpit yeast infection and Candida yeast infections
This article is for general educational purposes and is not medical advice. It does not replace an examination, and a new or changing body odor can have causes that need testing. Talk to a qualified healthcare professional before starting prescription antiperspirants or oral medications for sweating, particularly if you are pregnant, immunocompromised, taking prescription medication, or managing a diagnosed condition.
Frequently Asked Questions
Which race has the least body odor?
Body odor tracks a gene variant more than a racial category. One letter change (rs17822931) in the ABCC11 gene sets both earwax type and armpit odor. People with two copies of the A version secrete fewer odor precursors and have markedly less, though not zero, armpit odor. That version is more common in East Asian populations, which helps explain why a large share of people in the Far East produce little typical body odor. In a UK cohort, people with this genotype were also far more likely to skip deodorant.
How do you fix sour-smelling armpits?
A sour or acid-spicy smell comes mainly from short-chain fatty acids that armpit bacteria make, so the fix is the same as for other armpit odor: fewer bacteria and less trapped sweat. In studies, antiperspirant users had about 50 times fewer armpit bacteria and weaker odor than non-users, shaving made soap washing far more effective in men, and cotton shirts smelled better than polyester after exercise. If a sour smell is new and persistent, mention it to a clinician.
Is there a pill to stop body odor?
No pill targets odor-making bacteria. For heavy sweating, dermatologists can prescribe oral glycopyrronium or oxybutynin, which reduce sweat production throughout the body. They are used when other treatments fail, can cause dry mouth, constipation and blurred vision, and make overheating more likely, so they need a prescriber's supervision. Low-dose propranolol is sometimes taken before a stressful event by people who sweat mainly when anxious.
Does chlorophyll help with body odor?
The evidence is thin and old. The only study we read was a 1980 report of 62 nursing-home patients given chlorophyllin tablets, which described them as helpful for body and fecal odors. It had no control group, and the authors' own lab tests failed to confirm the antibacterial effect claimed for chlorophyllin. That is not enough to recommend it for armpit odor.
What is the old lady's smell called?
The odor associated with older people is often linked to 2-nonenal, a compound with a greasy, grassy smell. In a study of people aged 26 to 75, 2-nonenal was found only in those aged 40 or older. It forms when omega-7 fatty acids in the skin's surface oil break down by oxidation, and both those fatty acids and the oxidized lipids rose with age.
Is it bad to stop wearing deodorant?
It is not harmful, but expect your armpit microbiome to change. When people in one study stopped deodorant and antiperspirant, the number of culturable bacteria rose toward the levels seen in people who never use products. A Belgian study found each person's armpit community stayed stable on a steady routine and shifted distinctly when habits changed, so odor may vary for a while after any switch.
Does stress make armpits smell worse?
It can. When researchers put 40 teenagers aged 16 to 18 through a standardized social stress test, it produced large amounts of sweat and strong armpit odor, with higher odor scores in boys. Dermatologists sometimes use low-dose propranolol before events for people whose sweating is driven mainly by anxiety, and an antiperspirant helps with the wetness.
References
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