Period underwear is generally safe for most people, but the answer depends on what brand you choose and how you care for them. The two main concerns are chemical exposure (specifically a class of industrial chemicals called PFAS used in some moisture-resistant fabrics) and infection risk from prolonged wear. Neither issue is unique to period underwear, but both are worth understanding before you buy.
The PFAS Concern
PFAS (per- and polyfluoroalkyl substances) are synthetic chemicals used to make materials water-resistant and stain-proof. They’re found in nonstick cookware, food packaging, and some textiles. A class-action lawsuit against the brand Thinx alleged that their underwear contained PFAS, which brought the issue into public view. The concern isn’t theoretical: studies have linked PFAS exposure to period irregularities, ovarian disorders, high blood pressure during pregnancy, and lower birthweight in babies exposed in the womb.
What makes this particularly worrying for menstrual products is the location. The vulvar and vaginal area has higher absorption rates than regular skin, and experts at the Harvard T.H. Chan School of Public Health have flagged concerns about PFAS exposure during biologically vulnerable phases like puberty, pregnancy, and menopause. Since period underwear sits directly against this tissue for hours at a time, even low levels of PFAS could matter more here than in, say, a rain jacket.
Not all period underwear contains PFAS. Many brands have responded to the controversy by reformulating their products and marketing them as PFAS-free. If this concerns you, look for brands that provide third-party lab testing for PFAS and avoid products that advertise water-repellent or stain-resistant coatings without disclosing what chemicals achieve that effect.
Infection Risk Is Low With Proper Use
A common worry is that sitting in menstrual blood all day will breed bacteria and cause yeast infections or bacterial vaginosis. In practice, quality period underwear is designed with moisture-wicking layers that pull fluid away from the skin, keeping the surface relatively dry. This is similar in principle to how athletic wear handles sweat. The key is changing them within a reasonable window: roughly every 8 to 12 hours depending on your flow, similar to how often you’d swap a pad.
Unlike tampons, period underwear carries virtually no risk of toxic shock syndrome (TSS). TSS is linked to internal products that create a warm, oxygen-poor environment inside the body. A study published in eClinicalMedicine actually recommended sanitary pads or menstrual underwear over tampons for overnight use, especially for people who sleep longer than six hours, precisely because external products don’t carry that same risk.
Antimicrobial Treatments and Vaginal Flora
Some period underwear brands treat their fabrics with antimicrobial agents, often silver nanoparticles or ammonium quaternary compounds, to control odor and bacterial growth. This raises a reasonable question: could those antimicrobials disrupt the balance of healthy bacteria in and around the vagina?
A prospective study published in the Journal of Clinical Medicine tested antimicrobial underwear that used non-migrating technology, meaning the antimicrobial compound stays bonded to the fabric surface rather than leaching onto skin. The researchers found no disruption to the vaginal microbiota and no systemic absorption. The antimicrobial effect stayed local to the fabric. That said, not every brand uses non-migrating formulations, and cheaper products may use treatments that do transfer to skin. If you’re prone to sensitivity or recurrent infections, choosing brands that disclose their antimicrobial technology (or that skip it entirely in favor of untreated organic cotton linings) is a reasonable precaution.
They Hold Less Than You Think
One safety-adjacent issue is capacity. If you’re relying on period underwear as your only protection on a heavy day, you should know that lab testing tells a very different story than marketing claims. A study that measured the actual red blood cell capacity of menstrual products found that period underwear held an average of just 2 mL of blood. For comparison, a regular tampon holds about 20 mL, a heavy-day pad holds 31 to 52 mL, and menstrual discs hold an average of 61 mL.
That 2 mL figure means period underwear works well for light days, spotting, or as backup for a cup or tampon. On heavier flow days, relying on the underwear alone could mean sitting in saturated fabric for hours, which does increase the chance of skin irritation, odor, and discomfort. Pairing them with another product on heavy days is a practical solution.
Skin Irritation and Contact Dermatitis
The vulvar area is prone to contact dermatitis, and the Cleveland Clinic lists synthetic underwear materials (like nylon), dyes, and nickel among common triggers. Period underwear typically uses multiple layers of polyester, nylon, or spandex alongside a leak-proof membrane, often made of thermoplastic polyurethane (TPU). If you’ve ever had irritation from synthetic fabrics, you may react to certain period underwear brands.
Signs of vulvar contact dermatitis include itching, redness, burning, and swelling. If these symptoms appear after switching to period underwear, the product itself may be the cause. Brands that use organic cotton for the layer touching your skin tend to cause fewer reactions. Fragrance-free, dye-free options are also worth seeking out if you have sensitive skin.
No FDA Oversight (Yet)
Period underwear currently falls outside the FDA’s regulatory framework for menstrual products. A 2025 draft guidance document from the FDA covers tampons, pads, and menstrual cups but does not include period underwear. This means there are no standardized testing requirements for absorption claims, no mandated safety testing, and no labeling rules specific to these products. You’re relying entirely on the manufacturer’s own quality standards, which vary widely across the market.
How to Wash Them Safely
Proper washing matters both for hygiene and for the longevity of the product. CDC guidelines on laundering contaminated textiles note that detergent alone at room-temperature water (around 71 to 77°F) can reduce microbial contamination when combined with proper agitation and a laundry additive like oxygen-based bleach. Hot water at 160°F for 25 minutes provides a higher level of disinfection, but most period underwear care labels advise against high heat because it can degrade the leak-proof membrane.
A practical approach: rinse the underwear in cold water first to remove blood, then wash on a cold or warm cycle with detergent. Adding an oxygen-based (color-safe) bleach provides extra antimicrobial action without the fabric damage that chlorine bleach can cause. Skip the dryer and hang them to dry. This preserves the waterproof layer while still getting the fabric genuinely clean. If the underwear develops a persistent odor even after washing, that’s a sign bacteria aren’t being fully removed, and it’s time to replace the pair.
Choosing a Safer Option
- Check for PFAS-free certification. Look for brands that provide independent lab results, not just marketing claims.
- Prioritize cotton-lined gussets. Organic cotton against the skin reduces irritation risk compared to all-synthetic designs.
- Be realistic about capacity. Use them as backup or for light days, not as a standalone product for heavy flow.
- Replace worn-out pairs. Once the absorbent or waterproof layers start failing (leaking, persistent odor, pilling), the product is no longer performing as designed.
- Read the full materials list. Brands that disclose every layer’s composition give you more information to work with than those that simply say “moisture-wicking fabric.”