It wont hurt you, it's a little uncomfortable if you swallow enough (like gas) but it's not gonna make you sick or anything.

When I was 16, my friend and I had just eaten a big lunch and we were smoking a bowl of some pretty good weed, but I wasn't that experienced at the time. Well, I took a hit off the pipe and instead of inhaling, I swallowed a lot of my hit.

Well, five minutes later I burped and this huge cloud of smoke came out with my burp. It was awesome.