No actually the exact opposite! I use a Jax Shaker for my sons formula and it always washes perfectly clean. I have been using it for about six months now! I'm guessing if you let it sit with a shake in it for too long (other than in the fridge) without washing it, it could get into the plastic. But it really shouldn't be let to sit there anyway- it's made to be used right away and washed. If you aren't going to drink it right away, I'd put it into the fridge.