Your armpits smell because bacteria on your skin are feeding on your sweat and producing pungent waste products in the process. The sweat itself is nearly odorless when it first leaves your body. The smell only develops once specific species of bacteria break it down into volatile compounds, some of which smell like onions, cumin, or even rotten meat. Several factors determine how strong that smell gets, from your hormones and diet to the shirt you’re wearing.
How Bacteria Turn Sweat Into Odor
Your body has two types of sweat glands. Eccrine glands cover most of your skin and produce the watery sweat you notice during exercise or heat. Apocrine glands are concentrated in your armpits, groin, and a few other areas, and they secrete a thicker fluid directly into hair follicles. This apocrine sweat is the main raw material for body odor.
When apocrine sweat reaches the skin’s surface, bacteria that naturally live there immediately start breaking it down. Different bacteria produce different smells. Members of the Corynebacterium genus create fatty acids that smell goat-like or cumin-like. A species called Staphylococcus hominis produces a sulfur compound called a thioalcohol that gives armpits their characteristic rotten-onion or meaty smell. Your unique mix of bacterial species is a big reason why some people smell stronger than others, even with similar hygiene habits.
Why It Gets Worse at Certain Life Stages
Armpit odor typically starts at puberty, when rising hormone levels activate apocrine glands for the first time. Before puberty, those glands are essentially dormant, which is why young children rarely have noticeable body odor.
Hormonal shifts later in life can change your smell again. During perimenopause, dropping estrogen levels leave the body with relatively higher testosterone. That hormonal ratio can attract more odor-causing bacteria to sweat, making it smell noticeably funkier than before. Pregnancy, menstrual cycles, and certain hormonal medications can all shift the balance in similar ways.
Foods That Make It Worse
What you eat directly affects what comes out in your sweat. Cruciferous vegetables like broccoli, cauliflower, and cabbage are high in sulfur-containing compounds that break down into hydrogen sulfide, the same chemical responsible for a rotten-egg smell. Garlic, onions, cumin, and curry produce similar sulfur byproducts during digestion. These compounds don’t just show up on your breath. They react with sweat on your skin and amplify body odor.
Alcohol gets metabolized into acetate, a sweet-smelling compound that your body excretes through both your breath and your sweat. The more you drink, the more acetate you produce, and the stronger the effect. For people with trimethylaminuria (a metabolic condition covered below), even eating seafood can trigger a strong fishy odor within hours.
Your Clothes Are Trapping the Smell
If your armpits still smell bad even after showering, your shirt might be the problem. Research from the University of Alberta found that synthetic fabrics like polyester and nylon absorb significantly more odor-causing compounds than plant-based fabrics like cotton. Polyester repels water but loves oil, so it soaks up the oily, smelly byproducts of bacterial metabolism while letting the watery portion of sweat sit on the surface. Cotton and viscose do the opposite: they absorb the water and take in fewer of the stink-causing compounds.
This means a polyester workout shirt can develop a persistent smell that survives multiple washes, while a cotton shirt in the same situation stays relatively fresh. If you’ve noticed that certain shirts seem to “hold” odor no matter what you do, switching to natural fibers can make a real difference.
How to Reduce Armpit Odor
The most effective approach targets bacteria, moisture, or both. Antiperspirants containing aluminum compounds physically block sweat glands to reduce the amount of moisture bacteria can feed on. Clinical-strength formulas with higher aluminum chloride concentrations can reduce underarm sweat by roughly 75% in some cases. For best results, apply them at night when sweat production is lowest, giving the active ingredients time to form plugs in the sweat ducts before morning.
Antibacterial soaps and washes reduce the population of odor-causing bacteria on the skin. Shaving or trimming armpit hair also helps, since hair provides surface area for bacteria to cling to and traps moisture close to the skin. Wearing breathable, natural-fiber clothing gives bacteria less to work with throughout the day.
If over-the-counter options aren’t cutting it, the clinical term for persistent, unusually strong body odor is bromhidrosis. Apocrine bromhidrosis, the most common form, results from excessive apocrine secretion that becomes particularly foul once bacteria break it down. Prescription-strength topical treatments and other interventions exist for people whose odor doesn’t respond to standard hygiene measures.
When a Smell Change Signals Something Else
A sudden, noticeable shift in your body odor, especially one that doesn’t match your usual smell, can occasionally point to an underlying health issue. Liver or kidney disease can cause a bleach-like body odor as toxins that these organs normally filter start building up and get excreted through sweat instead. Uncontrolled diabetes can produce a fruity or acetone-like smell. Thyroid disorders and certain infections can also alter how you smell.
Trimethylaminuria, sometimes called fish odor syndrome, is a rare metabolic condition where the body can’t properly break down a compound called trimethylamine. People with this condition give off a strong fishy odor that can come from their skin, breath, or urine. The smell can range from barely noticeable to strong enough that others detect it from across a room. It often becomes more obvious during sweating or stress. Because people sometimes grow accustomed to their own scent, those with trimethylaminuria may not realize they smell unusual until someone else mentions it.
If your armpit odor has changed suddenly, smells unusual (bleach-like, fruity, or fishy rather than the typical musky or onion-like smell), or doesn’t improve with consistent hygiene changes, it’s worth getting checked out to rule out a metabolic or organ-related cause.