As much as I love pulling a fellow runner aside for a "butt check" on my aggressive flow days, it is SO NICE to have underwear like this as a back-up to catch anything that might shift or overflow from a pad. Plus, unlike your regular undies, you don't have to scrub the blood out of these — they're designed so it rinses right out in the sink with soap and warm water, and can get tossed in the washer and dryer from there.
Psst — reviewers love these undies, but suggest sizing up!
Promising review: "I deal with PCOS and endometriosis which make my periods HEAVY for the first 3-4 days. I ordered two pairs of these underwear. They absorbed better than a pad, were comfortable, and the top layer stayed dry. Amazing. I had been thinking about buying the really expensive version of these underwear, but the price was ridiculous. These underwear do exactly what I need them to do for half the price. They are easy to hand wash and hang dry. I plan on ordering more. The stitching and waistband are solid and they don't bunch, don't ride up, and don't roll. They provide front and back coverage. They're literally perfect. If you're on the fence about these, I urge you to give them a try. They'll save you money (no pads or tampons needed, and you won't stain your regular underwear and have to replace them)." —drsmaw
Get it from Amazon for $14.90+ (available in sizes XS-6XL, two colors, and in three-packs).