I used it for the first time this week after DS's 2 month shots, and noticed he was sleepier than normal. I don't know if that was from the shots or the Tylenol.
Tylenol shouldn't make your baby sleepy, it was the shots. I've never heard of side effects from Tylenol, other than being bad for the liver if it's overused or overdosed.
Re: does baby tylenol have any side effects?