Soak them in hydrogen peroxide. Watch as it bubbles, scrub at the stain for a while and then launder as usual.
I usually stain the sides of my underwear while I menstruate. What can I do?
Hydrogen peroxide mixed with cold water will remove period stains, you could also try baking soda.
Buy sanitary towels that have wings so they are more secure in your underwear.
Wear darker underwear during your period.