I take metro for GI issues. It takes 30-60 minutes to start working. I found that it works better for me, the longer I wait to eat after taking it, so I usually take it an hour before eating. Sometimes I forget and take it 30 minutes before eating. It still works. The longer you wait, the better it will work as it has more time to get into your system and start working. You can either take it 45 minutes or an hour before. 15 minutes isn't really going to make much of a difference as far as it's effectiveness goes. 45 minutes is more than enough time for it to start working.