Ah, okay. I didn't realize it was the rubber stopper being the culprit. I figured it was the plastic as I know certain plastics are attacked by some chemicals. I usually do the pull it up a bit thing anyway. I'll just pay more attention and not push it all the way down before I flush it out. Thanks!I'd say the glass ones would probably work best,but there are other options tone way is to use a disposable pipette,or you can still use a plastic syringe,but pull the plunger up a bit so the flavoring doesn't touch the black seal thing,that is what really gets affected by the flavoring is that black rubber stopper.