Thongs are not inherently unsanitary. The largest study designed specifically to test whether thong underwear increases infection risk found that wearing thongs was not an independent predictor of urinary tract infections, yeast infections, or bacterial vaginosis. The popular notion that a thin strip of fabric drags bacteria from the anal region to the vagina sounds intuitive enough, but the research that actually tests this idea has not been able to confirm it.
What the Research Actually Found
The study most directly relevant to this question is known as the B-THONG study, a large cross-sectional survey of nearly a thousand women seen at obstetrics and gynecology clinics. Researchers compared self-reported infection rates over the previous twelve months between women who regularly wore thongs and those who did not. On the surface, thong wearers did report somewhat higher rates of UTIs, yeast infections, and bacterial vaginosis in the raw numbers. About one in five thong wearers reported a UTI in the past year, compared with roughly one in nine among non-thong wearers, and the gap was similar for yeast infections.1Obstetrics & Gynecology. Brief vs Thong Hygiene in Obstetrics and Gynecology (B-THONG): A Survey Study
But here is where the story changes. When the researchers controlled for other factors, like sexual practices and underwear fabric, thong-wearing dropped out as a meaningful predictor. The only independent predictor of UTIs and bacterial vaginosis turned out to be oral sex. And the only underwear-related factor linked to yeast infections was wearing underwear with a non-cotton crotch, regardless of whether it was a thong, bikini, or brief.2PubMed. Brief versus Thong Hygiene in Obstetrics and Gynecology (B-THONG): A survey study The initial association between thongs and higher infection rates was a case of confounding: women who wore thongs also happened to differ from non-thong wearers in other ways that mattered more, including the fabric of their underwear and their sexual behavior.
The Bacterial Transfer Theory
The most common concern you hear about thongs is that the narrow strip of fabric acts like a wick, moving fecal bacteria from the perianal area toward the vagina and urethra with every step. This idea has been repeated so often by health columnists and even some clinicians that many people take it as established fact. It is not.
A dermatological study specifically tested whether string-style underwear increased contamination of vulvar skin by anorectal bacteria. The researchers measured vulvar skin temperature, humidity, skin surface pH, and aerobic bacterial counts in women wearing thong-style panties versus conventional underwear. The differences were negligible across every measure. Vulvar skin temperature was essentially identical between the two groups, and the bacterial populations found on vulvar skin did not differ in a way that supported the transfer theory.3PubMed. The vulvar skin microenvironment: impact of tight-fitting underwear on microclimate, pH and microflora A later review of feminine hygiene practices confirmed the same conclusion: the assumption that tighter underwear increases contamination from anorectal bacteria was not supported by the data.4PubMed Central. Feminine Intimate Hygiene: A Review of Healthy and Unhealthy Habits in Women
This does not mean bacterial transfer between the anus and vagina never happens. It clearly does, because the most common bacteria behind UTIs, E. coli, normally lives in the gut. The point is that underwear style does not appear to be the mechanism driving that transfer. Normal movement, wiping habits, sexual activity, and anatomy all play a role independent of what kind of underwear you have on.
Why Fabric Matters More Than Style
If you take one practical lesson from the research, it is this: the material your underwear is made from probably matters more than the shape it comes in. The B-THONG study found that wearing underwear with a non-cotton crotch was a significant predictor of yeast infections, even after adjusting for other factors.1Obstetrics & Gynecology. Brief vs Thong Hygiene in Obstetrics and Gynecology (B-THONG): A Survey Study The researchers specifically recommended that medical providers discuss underwear fabric rather than underwear style when patients are concerned about infections.2PubMed. Brief versus Thong Hygiene in Obstetrics and Gynecology (B-THONG): A survey study
The reasoning behind this is straightforward. Yeast thrives in warm, moist environments. Synthetic materials like polyester, nylon, and spandex trap heat and moisture against the skin more than cotton does, because cotton is more breathable and absorbs moisture away from the surface. A cotton-crotch thong is, by this logic, a better choice for vaginal health than a fully synthetic bikini brief. The problem is that many thongs on the market are made from lace, mesh, or synthetic blends with little to no cotton in the crotch area. If you are prone to yeast infections and wear thongs, checking the crotch material is a more evidence-based move than switching to a different cut.
Friction, Irritation, and Skin Reactions
Even if thongs do not increase infection risk, they can still cause discomfort, and the mechanism is simple: friction. A narrow band of fabric sitting in a crease of skin that moves constantly throughout the day can irritate the delicate tissue around the vulva and perineum. This is not unique to thongs. Any underwear that fits poorly, rides up, or rubs against sensitive areas can produce the same kind of irritation. But the design of a thong means there is less fabric to distribute pressure, so any rubbing tends to concentrate along a narrow line.
For most people, this friction is minor and causes no lasting issue. But for anyone who already has inflamed or irritated skin in the vulvar area, a thong can aggravate the problem. Conditions like contact dermatitis, folliculitis, or vulvar eczema can all be worsened by repeated friction from any tight garment. If you notice redness, itching, or small bumps along the line where the thong sits, the underwear is likely making an existing issue worse rather than causing a new one.
The fabric itself can also be a culprit separate from the fit. Textile dyes, particularly disperse dyes used in synthetic fabrics, are a known cause of allergic contact dermatitis. In documented cases, skin reactions appeared specifically where the garment pressed against or rubbed the skin, which is exactly what a thong does along the perineum and gluteal crease.5PubMed. Contact dermatitis from textile dyes Brightly colored synthetic thongs pose a somewhat higher risk for this than plain white cotton ones, simply because they are more likely to contain the types of dyes that trigger reactions. If you develop a localized rash that follows the outline of your underwear, a dye allergy is worth investigating.
Exercise and Sweat
One scenario where thong concerns come up frequently is during workouts. Sweating increases moisture in the genital area, and prolonged moisture promotes both yeast growth and skin breakdown. The question is whether a thong makes this situation worse than other underwear would.
Based on the microenvironment research, the answer is probably not by much. The study comparing string panties to conventional underwear found no meaningful difference in vulvar humidity or temperature between the two styles.3PubMed. The vulvar skin microenvironment: impact of tight-fitting underwear on microclimate, pH and microflora That said, the study was conducted under normal wearing conditions, not during intense physical activity. During exercise, the more practical concern is staying in sweaty underwear for hours afterward. Changing out of damp workout clothes promptly is a widely accepted hygiene recommendation regardless of underwear style. If you work out in a thong, the same rule applies: change when you are done. The style matters less than how long you sit in wet fabric.
Some athletic wear now comes with built-in moisture-wicking liners designed to be worn without underwear. Whether you prefer that approach or a separate thong underneath is largely a comfort question, not a health one, as long as the material against your skin breathes reasonably well.
When to Be More Cautious
While thongs are generally fine for most people, a few situations warrant extra thought. If you deal with recurrent yeast infections, the fabric guidance from the B-THONG study is worth taking seriously: stick with cotton crotch panels and avoid synthetic fabrics sitting against the vulva for extended periods.2PubMed. Brief versus Thong Hygiene in Obstetrics and Gynecology (B-THONG): A survey study This applies to all underwear, not just thongs, but since thongs are more commonly made from synthetic materials, the overlap is worth noting.
People with hemorrhoids or anal fissures may find that a thong’s back strip irritates already sensitive tissue. This is a comfort issue, not an infection issue, but it can slow healing if the area is constantly being rubbed. Similarly, anyone recovering from vulvar surgery or dealing with conditions like lichen sclerosus would benefit from underwear that minimizes contact and friction, which generally means something looser and fuller-coverage during the healing period.
Pregnancy is another time when some women rethink their underwear choices, though not necessarily for infection-related reasons. Increased vaginal discharge during pregnancy means more moisture against the skin, and some pregnant women find that a wider cotton gusset keeps them more comfortable. The B-THONG study did not specifically enroll pregnant women, so its findings cannot be directly generalized to that population, but the basic fabric principles still hold.
What Doctors Actually Advise (and Why It Sometimes Conflicts)
If you have ever been told by a doctor to stop wearing thongs, you are not alone. Many gynecologists and nurse practitioners have offered this advice for years, and it often comes from a reasonable-sounding theory about bacterial transfer. The issue is that the theory was not rigorously tested until relatively recently, and the results did not support the recommendation.
The B-THONG researchers specifically noted that medical providers should be discussing sexual practices and underwear fabric with patients concerned about urogenital infections, rather than focusing on underwear style.2PubMed. Brief versus Thong Hygiene in Obstetrics and Gynecology (B-THONG): A survey study This is a meaningful distinction. Telling someone to ditch their thongs gives them the impression they have addressed a risk factor, when the evidence says they have not. Meanwhile, the actual predictors, sexual practices and fabric choices, go unmentioned.
This is not to say that any doctor who recommends switching away from thongs is wrong in every case. If someone presents with vulvar irritation concentrated along the line where a thong sits, recommending different underwear is perfectly reasonable. The issue is with blanket advice that treats thong-wearing as a general infection risk. That advice is not supported by the available data, and it can distract from interventions that would actually help.
Hygiene Practices That Actually Matter
Given that the research consistently points away from underwear style as a meaningful health factor, what does make a difference for genital health and hygiene? The evidence and standard gynecological guidance converge on a few practices:
- Cotton crotch: If you are prone to yeast infections, choosing underwear with a cotton crotch panel can reduce your risk regardless of whether the garment is a thong, bikini, or brief.
- Change after sweating: Sitting in damp underwear for hours creates the warm, moist environment that yeast and bacteria love. Changing after workouts or on particularly sweaty days is a simple and effective step.
- Avoid scented products: Scented detergents, dryer sheets, vaginal washes, and sprays are more likely to disrupt vaginal pH and cause irritation than any underwear style.
- Proper sizing: Underwear that is too tight can cause friction and pressure regardless of its design. A well-fitting thong will generally cause less irritation than an ill-fitting brief that bunches and digs in.
None of these guidelines single out thongs as a unique problem. They apply to all underwear, and they address the factors that research has actually linked to infections and irritation.
How the “Thongs Are Dirty” Myth Persists
The persistence of the idea that thongs are unsanitary is a case study in how medical advice can outrun the evidence. The bacterial transfer theory is plausible enough on its face that it became clinical folklore before anyone tested it. Once something enters the informal consensus of clinical practice, it takes strong counter-evidence to dislodge it, and even then, the old advice lingers in patient handouts, blog posts, and casual exam-room recommendations for years.
Social and cultural factors also play a role. Thongs have been wrapped up in broader conversations about modesty, sexuality, and self-presentation since they became mainstream in the late 1990s. Health concerns can serve as a socially acceptable way to express disapproval of a garment that some people find provocative. This does not mean every person who raises health concerns about thongs is being judgmental, but it does mean the topic carries cultural baggage that can amplify unverified claims.
The research base on this subject remains relatively small. The B-THONG study and the vulvar microenvironment study are among the only investigations to directly test the thong-specific health question. There is no shortage of studies on underwear fabric and vaginal health more broadly, but the specific claim that a thong’s shape creates a unique bacterial pathway has attracted remarkably little formal investigation given how widely it has been repeated. That said, both studies that did test the claim reached the same conclusion: the shape of the underwear is not the problem. What is in contact with your skin, how long it stays moist, and what else is going on in your sexual health are far more relevant to whether you get an infection than whether your underwear has full rear coverage.