Cotton vs synthetic underwear: what's best for hygiene, pH and all-day wear?

Short answer: for hygiene and pH, it is not the whole garment that matters, it is what sits against you and how dry it stays. Cotton is excellent right against the skin because it is breathable and absorbent, which is why the gusset should always be cotton-lined. But for the rest of the pair, a breathable moisture-wicking knit can keep you drier than damp cotton, especially when you are active. The healthiest design is not cotton-everything or synthetic-everything. It is a cotton-lined gusset with a breathable body.

This gets over-simplified constantly online, usually by someone with something to sell. Here is the honest version.

Does synthetic underwear affect your vaginal pH?

The thing that disrupts the natural balance is a warm, damp, trapped environment and friction, not the name of the fibre. There is no strong evidence that a synthetic fibre chemically changes your pH on its own. What causes trouble is fabric that holds moisture and heat against your skin. A non-breathable material sitting against damp skin is the real problem, and that can be a cheap airtight synthetic or a soggy pair of wet cotton. So "synthetic is bad for your pH" is too blunt to be true. What actually matters is staying dry and breathable.

Is cotton or natural fibre best "down there"?

For the part in direct contact, the gusset, yes. Cotton is soft, breathable and absorbent, which is exactly what you want against the most sensitive skin. That is why any well-made pair, including performance underwear, lines the gusset in cotton. For the body of the garment, away from that area, breathability and staying dry matter more than the specific fibre. Natural does not automatically mean healthier if it stays wet all day.

Cotton vs synthetic for working out

Here is the counterintuitive part that surprises people. When you sweat, cotton absorbs moisture and then holds it, staying wet against you, which creates the exact warm, damp environment you want to avoid. A moisture-wicking knit moves sweat away from the skin and dries quickly, so you stay drier. For a workout, a breathable wicking fabric with a cotton-lined gusset beats an all-cotton pair that turns soggy and stays that way. Dry is the goal, and cotton is not always the fabric that keeps you driest.

What about all-day wear?

Breathability and dryness again, plus fit. A cotton-lined gusset with a breathable knit is comfortable and healthy for a normal day, as long as you change your underwear daily. Friction matters just as much as fibre: a pair that digs or rubs irritates skin regardless of what it is made of. So the all-day answer is the same, a breathable pair that fits and stays dry, with cotton where it counts.

So what is the best underwear for hygiene and comfort?

The best-of-both design: a cotton-lined gusset for the skin that needs it, and a breathable, moisture-managing body that keeps you dry the rest of the time. Change it daily, wear your size, and avoid anything that traps humidity or rubs. That combination covers everything the evidence actually points to. It is exactly how we build the Anywhere G: cotton-lined gusset, breathable recycled performance knit, bonded flat edges so nothing rubs.

The honest bottom line

It was never cotton versus synthetic. It is dry-and-breathable versus damp-and-trapped. Put cotton where it matters most, the gusset, use a breathable knit that keeps you dry for the rest, fit it properly and change it daily, and you have done everything that genuinely supports hygiene and comfort. If you are dealing with recurrent irritation, that is worth a conversation with your GP rather than the internet. For the everyday case, this is the whole answer. We lay out the evidence in more depth on the science page.

One seamless G-string built on exactly this principle, from $21.90 a pair in packs.