An egg-like or sulfurous vaginal odor usually comes from a shift in the balance of bacteria in your vagina. The most common cause is bacterial vaginosis (BV), a condition where certain bacteria overgrow and produce sulfur-containing compounds that smell like eggs or, more classically, fish. Other possible causes include sweat breakdown in the groin area, a forgotten tampon, or less commonly, a sexually transmitted infection like trichomoniasis.
How Your Vaginal Microbiome Creates Odor
A healthy vagina is dominated by beneficial bacteria called lactobacilli, which typically make up more than 70% of the vaginal microbiome and in some cases are practically the only bacteria present. These bacteria keep the vaginal pH acidic, in the range of 3.8 to 4.5, and produce antimicrobial substances that prevent harmful organisms from taking over.
When something disrupts that balance, other bacteria multiply and start breaking down proteins and amino acids in vaginal fluid. This process releases amine compounds, the same type of chemicals responsible for the smell of rotten eggs, fish, or other strong odors. The higher your vaginal pH climbs above 4.5, the more noticeable these compounds become.
Bacterial Vaginosis: The Most Likely Cause
BV is the single most common reason for unusual vaginal odor in women of reproductive age. It happens when bacteria that are normally present in small numbers, particularly Gardnerella vaginalis, overgrow and crowd out the protective lactobacilli. These overgrown bacteria produce enzymes that break down proteins and convert amino acids into foul-smelling amines.
The classic BV smell is often described as fishy, but many women experience it as more eggy or sulfurous, especially depending on which specific bacteria dominate the overgrowth. The odor tends to be stronger after sex and during your period, because semen and menstrual blood both raise vaginal pH and release more of those amine compounds. You may also notice a thin, off-white, grayish, or greenish discharge, though some women with BV have no symptoms beyond the smell.
BV is not a sexually transmitted infection, but sexual activity can trigger it. Douching, new sexual partners, and anything that disrupts your vaginal environment can tip the bacterial balance. It’s treated with prescription antibiotics, either oral or vaginal, and typically clears within a week of starting treatment.
Sweat and Skin Bacteria
Your groin has a high concentration of apocrine sweat glands, which produce a thicker type of sweat than what you get on your arms or forehead. This sweat itself is nearly odorless when it first leaves the gland. The smell develops when bacteria on your skin break it down, producing compounds that can range from musky to sulfurous.
If the egg-like smell seems to come from the outer vulvar skin rather than from vaginal discharge, sweat is a likely explanation. This is especially true after exercise, on hot days, or if you’ve been wearing tight, non-breathable clothing. Washing the vulva with warm water (no soap inside the vagina) and wearing breathable cotton underwear typically handles this. If the odor persists despite good hygiene, the source is more likely internal.
A Forgotten Tampon or Other Object
A retained tampon is a surprisingly common cause of sudden, strong vaginal odor. When a tampon stays in too long, the warm, moist environment allows your normal vaginal bacteria to overgrow rapidly, which can trigger BV or produce its own intense smell. The odor is often described as rotten or decaying and tends to get worse the longer the object stays in place.
If you notice a sudden, unusually strong smell and can’t identify a cause, it’s worth checking whether a tampon, menstrual cup, or piece of a condom may have been left behind. Removing the object usually resolves the odor within a day or two, though you may need antibiotics if an infection developed in the meantime.
Trichomoniasis and Other Infections
Trichomoniasis is a sexually transmitted infection caused by a parasite. It can produce a fishy or sulfurous vaginal odor along with a thin discharge that may be clear, white, yellowish, or greenish. Many people with trichomoniasis also experience itching, burning, or irritation in the vaginal area. It’s treated with a single course of prescription antibiotics, and sexual partners need treatment too to prevent reinfection.
Yeast infections, by contrast, don’t typically cause a strong odor. If you have a thick, white, cottage cheese-like discharge with itching but no significant smell, that’s more consistent with yeast than with the conditions that produce an egg-like odor.
Less Common Causes
In rare cases, unusual vaginal odor can signal something more serious. A rectovaginal fistula, which is a small opening between the rectum and vagina, can allow gas or stool to pass into the vaginal canal, producing a sulfurous or fecal smell. This is uncommon and usually follows childbirth injury, surgery, or inflammatory bowel disease. Cervical and vaginal cancers can also produce unusual odor, though this is rare and typically accompanied by other symptoms like abnormal bleeding.
Signs That Need Medical Attention
A mild change in vaginal odor around your period or after exercise is normal and doesn’t necessarily require a visit to your doctor. But certain symptoms together point toward an infection that needs treatment:
- Greenish, yellowish, thick, or cheesy discharge
- A strong odor that doesn’t go away with normal hygiene over a few days
- Itching, burning, or irritation of the vagina or vulva
- Redness or color changes to the vulvar skin
- Bleeding or spotting outside your normal period
BV and trichomoniasis both respond well to antibiotics, but they won’t resolve on their own. Getting the right diagnosis matters because the treatments are different, and an untreated infection can increase your risk of complications over time.