Tylenol is definitely toxic to both dogs
and cats. In fact, some of us humans can't even take the stuff. Too much is toxic for
our livers, so one can only imagine why it's so harmful to our much smaller dogs and cats. Plus, dogs and cats metabolize Tylenol much differently than humans do, which is why it can kill them.
Care has to be taken with Tramadol, too. I ended up being allergic to it myself, so I would say it's possible for animals to be allergic to medications, too.