What is Period Underwear? Period Care Founders Share What You Should Know

If you’ve ever taken a stroll down the period-care aisle at your local drugstore (or spent any amount of time shopping for intimates online), you’ve likely encountered period underwear. But what is period underwear, really? The reusable alternative, which can look almost identical to a regular pair, is made with absorbent layers designed to collect menstrual blood and help protect against leaks.

Period underwear can be worn on its own or as a backup with another period product, and depending on what you prefer, it comes in everything from thongs and briefs to boyshorts and high-waisted cuts that offer more coverage. Ready to learn more about period underwear? Keep reading to discover how period underwear works, how long you can wear it, and how to wash and care for it.

---

Meet the Experts

Nina Smith is the founder of Fri Period, a period underwear brand she launched in 2023 after wanting better options for managing periods. She helped develop the brand around period underwear that is made free from PFAS, designed for leak protection, and produced through a transparent supply chain.

Arielle Loupos is the founder of Flower Girl, a Los Angeles-based period underwear brand. She spent two years researching materials, testing fabrics, and working with local manufacturers before launching the company.

---

How Does Period Underwear Work?

The secret is in the gusset (the panel of fabric that sits between your legs). “Period underwear looks and feels like regular underwear, but it’s built with layers of absorbent fabric sewn right into the gusset,” Smith explains. Instead of putting a pad on top or inserting another period product, those layers are already part of the underwear.

Loupos describes the construction as a three-layer system. The top layer helps pull moisture away from your skin, the middle layer absorbs your flow and helps control odor, and the bottom waterproof layer “holds the flow in the underwear.” Smith explains it similarly: liquid moves through a moisture-wicking layer, gets soaked up by an absorbent core, then meets a leakproof barrier designed to keep it from reaching your clothes. That’s what lets you bleed directly into the pair.

How Absorbent Is Period Underwear?

Absorbency varies by brand and pair, which is why you’ll usually see options made for light, moderate, or heavier flow. Some are better suited to backup protection, while others are designed to replace a pad or tampon altogether.

Fri Period’s boxer briefs, for example, are designed to absorb the equivalent of “4 regular tampons or pads,” Smith says. She adds that plenty of people wear period underwear as their only period product from start to finish. Others prefer using it as backup on heavier days, overnight, or at the beginning and end of a cycle when flow can be less predictable.

“There’s no ‘right’ way to use it,” Smith points out. “It’s really about what makes your period feel easiest to manage.”

How Long Can You Wear Period Underwear?

How long you can wear one pair mostly comes down to your flow and the absorbency level you chose. NIXI Body co-founder Kelly Newton, who spoke with us in more detail about how long you can wear period underwear, says there isn’t “a single number that works for everyone.”

There is a helpful upper limit, though. Newton says that when you’re wearing the right absorbency for your flow, period underwear can generally be worn for up to 12 hours. “The key is matching the pair to the day,” she explains. A lighter day may mean one pair carries you through from morning to night, while heavier days can call for a change or two along the way.

If you’re new to period underwear, keeping a spare pair in your bag can be reassuring on days when your flow is harder to predict. “My advice is always to choose your absorbency based on where you are in your cycle, and to keep a spare pair with you on your heaviest days. Get the absorbency right and you’ll get the full wear out of each pair,” Smith recommends. After a few wears, you’ll usually get a better sense of what works for your flow and when you need a new pair.