I just bought some protein powder today, and it seems it has expired. I don't have the receit so I can't return it. It's been expired for 3 months. What happens if protein powder is expired??

what happens is that gullible people think that it doesn't work anymore and they go out and buy more. that's the companies way of making more money.

i doubt the protein has denatured to any measurable extent.

there is a "shelf life " beyond the expiration, just like when you buy milk, it tastes fine for a few days after the date. for dry goods, that time is much longer, but who can say how long exactly that is. mfr's don't publish it - if it mixes fine/tastes fine, i'd use it up.