The thing about soda, is when it fizzes, those bubbles can cause gas. In Dean or any of us, we just burp. If Sam doesn’t let the fizz settle and drinks it really fast, that’ll hurt him a lot. Those are big bubbles, very little tummy. He got a stomach ache for the night, and Dean wouldn’t leave him alone, mostly panicking because they have no idea how medicine would affect Sam.
( From Family Movie Night 2 )