Knix Leakproof High Rise Period Underwear Review 2023

The Knix Leakproof High Rise Period Underwear came out as the top pick for moderate to light flow days in my larger tested best period underwear review. Ahead, learn why this pair is a smart alternative to tampons and pads.

Period underwear offers a big promise: leakproof underwear that can absorb liquid from your menstrual cycle and can be used over and over again to make your period manageable and sustainable. As more and more brands enter the menstrual underwear space, Knix continues to stand out as a high-quality, comfortable and safe option.

After months of testing, I determined that Knix's Leakproof High Rise briefs are the best period ... [+] underwear for moderate flow days.

The Knix Leakproof High Rise underwear was my favorite option for low to moderate flow days. The underwear is comfortable, seam-free (no underwear lines!) and absorbent. You can also trust that Knix products aren’t made with PFAS, a group of chemical compounds that have raised concern in recent years. Read on to learn about why it’s one of my top picks.

Knix

Materials: Nylon, Lycra, cotton, spandex | Absorbency: Low to moderate | Sizing: XS to XXXXL | Fit: High rise

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The Knix Leakproof High Rise underwear is high-cut—it rested just above my bellybutton, at my natural waist—and is also seam-free, which means you can’t see underwear lines (even while wearing leggings). Overall, it has less of a diaper-like feel compared to many of the other options I tried. In fact, I’d say this pair was an almost sexy option; the briefs made me feel held in, and I nearly forgot that I was wearing them, which is ideal.

Knix are made with a mix of fabrics, including nylon, Lycra, cotton and spandex. The Lycra, especially, aids in the slimming effect. But these are also softer than almost any other underwear I tried, apart from the Saalt Leakproof Comfort Brief, and they didn’t cut into my legs or my waist. If you don’t like the high-rise fit, Knix offers plenty of other cut options, ranging from bikinis to boyshorts to thongs. Their overnight undies also receive praise from online reviewers.

I wore the Knix Leakproof High Rise underwear on a heavy day and found myself disappointed by the absorbency. They’re advertised as able to absorb 3 teaspoons of liquid, but I found it to be even less. After 3 hours, I started to experience leaks from the front of the gusset, which indicated that the absorbency had maxed out at around one tampon’s worth of fluid. Based on that, I tried these during a moderate flow day and was far more pleased. Use them on a lighter day, or as a backup (with a tampon) on a heavy day, and you’ll definitely get enough coverage.

I was pleased to find that these remained absorbent even after four rounds in the wash. You can machine-wash them, but the brand recommends laying them flat to dry.

Unlike many other brands, Knix is transparent about their PFAS test results. They’ve been sharing results since 2020, and you can always find their newest testing info on their blog. They promise that their products are made without PFAS. (PFAS are in the water and the environment, so they also explain on their blog that they need to double-check for PFAS at every step of the process, holding their suppliers and manufacturers accountable as well.)

Knix underwear is more inconspicuous than the other underwear I tried; it’s seamless and doesn’t feel like a diaper, which I loved. It’s also sexier than any of the other pairs I tried, with a slimming effect that was far cuter than other pairs I liked, including the Aerie Real Period Boybrief Underwear. They’re as soft as the Saalt Leakproof Comfort Brief and less absorbent than the Saalt and Aerie undies, but ideal for a low to moderate flow day.

For this guide, I spent several hours doing internet research to narrow down my options. I compared more than 20 options across multiple brands and opted to focus my testing on classic cuts that ranged from light absorbency for lighter flow days, up to the highest absorbency options for heavier flow days or overnight use.

I tested a variety of period underwear for my review, and found that Knix came out on top for light ... [+] to moderate flow days.

After narrowing my list, I spoke to a few experts about the science behind period underwear, including Dr. Stephanie Hack, MD/MPH, OB-GYN and founder of the Lady Parts Doctor women’s health platform; and Dr. Jon Chorover, a professor of environmental chemistry and the head of the Department of Environmental Science at the University of Arizona.

I also spent a fair bit of time researching the recent class action lawsuit against Thinx. In short, some pairs of Thinx underwear (and underwear from other major brands) were found to include PFAS, which are human-made chemicals that have been linked to adverse human health effects. PFAS help products repel moisture, and you’ll find them in nonstick pans, raincoats and beyond. But, Dr. Chorover says, you don’t want to find PFAS in a product that sits right next to an absorbent part of your body—which applies to menstrual products. I spoke with Knix representatives (after I finished my testing, to avoid bias) to understand their protocols related to PFAS avoidance. In short, you don’t need to worry about finding PFAS in their products.

I then ordered seven pairs of period underwear and wore them each at least once during two of my cycles. I tried the Knix Leakproof High Rises during heavy days and lighter days. I also washed them four times, according to their instructions.

I’m a freelance journalist who’s been writing product reviews, parenting content and more for over a decade. I have a master’s degree in science journalism and love reporting on health-based issues. Before I started freelancing, I worked as a full-time editor at Wirecutter, where I reviewed and wrote about outdoor and travel products. You can find my bylines in the New York Times, Forbes, Outside magazine, Slate and beyond. Most of my reviews these days are focused on health, the outdoors and baby and kid gear.

I’m an active person—I hike, run and practice yoga daily—and I’m also a mom to two kids. I recently tested diapers for Forbes, which gave me a good bit of knowledge about absorbency. As it applies to this guide, my menstrual cycle only recently came back after breastfeeding my now 1-year-old daughter. I’ve always had a mild period, with 28-day cycles that come and go like clockwork. But post-kids, my period is heavier and tends to be more irregular. I stopped taking hormonal birth control (which had eliminated my period) nearly a decade ago and never looked back.

Yes. Check the absorbency information on the underwear you purchase. Knix Leakproof High Rise underwear can absorb 3 teaspoons of fluid. It doesn’t matter what fluid you want to catch; many people use period underwear to absorb sweat, urine and menstrual blood. Postpartum parents, in particular, may turn to period underwear for absorbency while rebuilding pelvic floor stretch, especially during exercise.

Yes. Although they may feel thicker than you prefer, you can wear period underwear anytime.

Read the manufacturer’s instructions. Most brands suggest line drying period underwear to help them hold up for longer; the dryer can cause materials to break down more quickly, and may mean less absorbency over time. Knix suggests machine-washing and laying flat to dry.