Wearing underwear is not bad for you. For most people, it’s a neutral or even protective habit, shielding skin from friction against outer clothing and absorbing sweat throughout the day. The real health questions come down to what kind of underwear you wear, what fabric it’s made from, and how long you keep it on, not whether you wear it at all.
Fabric Matters More Than the Underwear Itself
Cotton is the gold standard for underwear fabric. It’s breathable, wicks away moisture, and is less likely to trigger allergic reactions than synthetic materials. The American College of Obstetricians and Gynecologists specifically recommends cotton underwear because it lets the genital area breathe more than synthetic fabrics. The Cleveland Clinic echoes this, noting that excess moisture trapped against the skin creates an environment where bacteria and yeast thrive.
Synthetic fabrics like polyester and nylon hold heat and moisture closer to the body. For people with vulvas, this can contribute to yeast infections, bacterial vaginosis, and vulvar irritation. If your underwear is synthetic but has a cotton crotch panel, that small strip doesn’t fully protect you. It won’t breathe the way a fully cotton pair would.
Elastic waistbands and dyed fabrics can also cause problems for some people. The chemical additives used in manufacturing latex and elastic components can trigger allergic contact dermatitis, a delayed skin reaction that shows up as a blistering rash 24 to 48 hours after exposure. If you notice itching or irritation that follows the line of your waistband or leg openings, the elastic or dye could be the culprit rather than the underwear concept itself.
Tight Underwear and Sperm: What the Evidence Shows
One of the most persistent health concerns about underwear is that tight briefs overheat the testicles and lower sperm count. The logic makes sense on the surface: scrotal temperature runs about 1.5 to 2 degrees Celsius below core body temperature, and sperm production is sensitive to heat. But the actual evidence doesn’t support the concern.
One of the highest-quality studies comparing boxers to briefs measured scrotal temperature directly and found no difference between the two groups. Sperm counts weren’t different either. So while prolonged heat exposure from other sources (like hot tubs or laptop use) can affect sperm production, your choice between boxers and briefs is unlikely to matter for fertility.
Thongs and Bacteria Transfer
Thong underwear gets singled out for a specific concern: the narrow strip of fabric sits between the anal and vaginal areas, and the worry is that it acts as a bridge for bacteria, increasing the risk of urinary tract infections or vaginal infections. A survey study of nearly 1,000 women published in Obstetrics & Gynecology investigated exactly this. At first glance, thong wearers did report higher rates of UTIs (20.4% vs. 11.4%), yeast infections (22.4% vs. 15.6%), and bacterial vaginosis (9.7% vs. 5.7%) compared to women who never wore thongs.
But when researchers controlled for other variables, the thong itself dropped out as a risk factor. Sexual behaviors and hygiene choices, not underwear style, predicted infection rates. The one underwear-related finding that held up was wearing non-cotton crotch fabric, which was linked to a higher risk of yeast infections. So the fabric, again, matters more than the cut.
Going Without Underwear at Night
Sleeping without underwear is one scenario where going commando has a genuine advantage. During sleep, your body doesn’t need the friction protection that underwear provides during the day, and going without gives skin a chance to air out after hours of moisture buildup. Gynecologists at the University of Wisconsin-Madison have noted that sleeping without underwear can be “really helpful” because it alleviates friction-related and moisture-related damage to vulvar skin.
This is particularly useful if you’re prone to recurring yeast infections or vulvar irritation. Giving the area several hours of uninterrupted airflow each night can help keep the skin dry and reduce the conditions that promote fungal overgrowth. For people who prefer to wear something to bed, loose-fitting cotton shorts or pajama pants without underwear underneath achieve a similar effect.
Exercise and Moisture Management
During physical activity, the equation shifts. Working out without underwear increases direct friction between skin and outer clothing, which can cause chafing on the inner thighs, groin folds, and buttocks. Moisture-wicking athletic underwear is designed to pull sweat away from the skin and move it to the outer layer of the fabric, where it evaporates. This keeps skin drier and reduces the risk of chafing and a condition called intertrigo, where warm, moist skin folds become irritated and susceptible to fungal growth.
The key after exercise is changing out of sweaty clothing promptly. ACOG recommends changing out of wet clothes after swimming or working out rather than staying in damp fabric. Sitting in moist underwear for hours post-workout creates the same trapped-moisture problem that synthetic fabrics cause during everyday wear.
Practical Takeaways by Situation
- Daily wear: Cotton underwear in a comfortable fit is the safest default. Avoid all-synthetic fabrics if you’re prone to irritation or infections.
- Sleep: Going without underwear gives skin time to breathe and can reduce recurring irritation or infections.
- Exercise: Moisture-wicking underwear with minimal inner-thigh seams reduces chafing. Change out of it soon after finishing.
- Fertility concerns: Underwear style (boxers vs. briefs) does not meaningfully affect sperm count based on the best available evidence.
- Skin reactions: If you notice rashes along waistband or elastic lines, try underwear with different dyes or latex-free elastic to rule out contact dermatitis.