Period underwear, explained: reusable and disposable

Period underwear looks like the underwear in any drawer. The difference is inside, in the layers. It covers reusable and disposable pairs. It describes; it does not rank either kind, or any other format. Two people with the same flow choose differently, and both are right.

What it is

The NHS says period underwear will “soak up blood in the same way as pads”. A clinical brief report describes it as “available as reusable or disposable products resembling normal underwear with built in absorbency”. In the US, the FDA puts it in the same group as period pads: “Menstrual pads, which include liners or period underwear, absorb menstrual flow after it leaves the body.”

So period underwear absorbs. The blood is held in fabric layers built into the pair. Cups and discs work differently: they collect.

Reusable pairs are washed and worn again. Disposable pairs are worn once. How much each format holds, and why the numbers differ, is in the Column “‘Super’ on a period pad and ‘Super’ on a tampon are not the same thing.”

A note on words: some sources we quote here say “pads”, and one US source is the Office on Women’s Health. We keep sources’ words so we report them accurately. In our own words, we say “period pads”. The Reading Room is for everyone who menstruates.

How it’s used

Putting it on. A pair is worn in place of regular underwear. Special steps for putting it on are not described in the sources we checked.

Wear time. The sources differ, so here is who says what. The US Office on Women’s Health says period underwear “can usually last about a day, depending on the style and your flow”. SickKids says it “can be worn all day for up to 24 hours, depending on your flow”, and that it is “best for times of light bleeding and can be used in combination with other menstrual products during heavier flow”. The maker’s instructions for your pair come first.

Reading a size chart. One Canadian maker’s size chart asks for hip and waist measurements in centimetres. The same maker says: “Our sizing can vary slightly by style, so we recommend checking the chart for each product.” In forums, readers report that one hip measurement can map to different sizes across makers. So the step the sources support is simple: measure, then read each maker’s chart for each style.

Coverage. Styles differ, and clinicians note that the number of absorbent layers “varies by and within brands”. The maker’s product page is where a style’s coverage is described, when the maker gives it.

Measure, then read each maker’s chart for each style.

What it’s made of

Clinicians describe the inside of period underwear as “multiple layers of polyester designed to wick moisture away from the skin, with an outer liquid-repellant layer of nylon and lycra.”

To wick is to draw moisture away from the skin. Clinicians describe the outer layer as “liquid-repellant”.

In Canada, a reusable pair must carry a fibre-content label, which lists the fibres it is made of. That label is the most direct source for which fibres a pair is made of. It does not list finishes or treatments.

What disposable pairs are made of is not described in the sources we checked.

Periodica makes no judgement about materials. This Column names them as the sources name them.

Care or disposal

Reusable pairs. The Reading Room has a full Column on this: “How to wash period underwear, any brand”. In short, the makers’ care pages mostly agree: rinse in cold water, wash gently, leave out the fabric softener. Then they split on the dryer.

Canada’s Department of National Defence says to rinse reusable period underwear by hand “until the water runs clear”, then wash it and let it dry before wearing it again. Your pair’s own care label comes first.

Disposable pairs. A disposable pair is worn once. Disposal steps specific to disposable period underwear are not described in the sources we checked. The Department of National Defence says: “No period products should be flushed down a toilet.” Garbage and green-bin rules differ by city, so your city’s waste page is the place to check.

What the label tells you (in Canada)

Here is what Canadian rules say about period underwear, in the rule-makers’ own words.

Period underwear falls under consumer-product safety law and textile labelling law. A reusable pair is a textile article worn on the person, so fibre-content labelling applies. Disposable articles are exempt when they are “intended for a one-time use only”.

There is no Canadian absorbency standard for period underwear. Labels describe absorbency in each maker’s own units, often “tampons’ worth”. No public standard sits behind those units.

In the US, the FDA regulates all menstrual products as medical devices, and groups period underwear with period pads. Its draft guidance (October 2025) places period underwear under the rule for unscented menstrual pads, a Class I device.

Common questions

Does it feel wet? Clinicians say the layers are “designed to wick moisture away from the skin”. How that feels to wear is personal, and it is not described in the sources we checked.

What sizes exist, including plus sizes and petite or teen sizes? Ranges differ by maker. One Toronto maker’s listings run from XS to XXXXL, and it keeps a separate teen range. In a UK consumer test, the tested ranges ran as wide as UK 4 to 22, UK 6 to 24, and XS to 3XL. In forums, some readers report difficulty finding sizes small enough for younger or smaller bodies. We found no source on the smallest sizes sold in Canada.

What is disposable period underwear? A pair worn once, then thrown away. It resembles regular underwear, with absorbency built in. Because it is intended for one-time use, it is exempt from fibre-content labelling.

Troubleshooting

Leaking at the sides. Coverage differs by style, and the number of absorbent layers “varies by and within brands”. One maker says its sizing “can vary slightly by style”, so a size that fits in one style may sit differently in another.

A smell after washing. The care Column covers this. One maker links smell to timing: blood left to dry in the fabric can become stale, so it suggests rinsing and washing soon after wear. If a smell stays after washing, the Editor did not find a source that explains why. Check your maker’s care guidance, or contact the maker.

Between two makers’ charts. A size letter is not a measurement. Read each maker’s chart in centimetres, for the style you are looking at.

What this is not

This Column described period underwear in the words of clinicians, health bodies, makers and Canadian law. It did not compare brands or rank formats. It makes no claim about any product’s materials, and no claim that any product contains or is free of PFAS. The test findings quoted are Which?’s, about products it tested in the UK. It did not tell you which format to choose.