I put frontline on my dog, he then scratched his back where the frontline was and licked the paw he scratched with. Could this cause vomiting? or other symptoms ?

Don't worry about it unless he ate a whole thing which he didn't. I mean the front line company would be well aware of the fact that dogs might lick the frontline so i'm sure it isn't toxic stuff but to avoid him licking it you should put it near his head on the neck so that he can't reach it.:)

If he licks enough of it, YES it can make him sick or even worse.

By your description, it sounds like you're not applying it in the proper location.

you should apply Frontline or Advantage or any of these applications between their upper shoulder blades and do so before bedtime. At that location they can't reach it with any part of their body and once they're off to bed for the night they won't even be trying.

In large enough amounts, it could cause excessive salivation, vomiting, and seizures. However, the tiny dose you describe won't hurt him.

As long as he isn't allergic and is otherwise healthy, he'll be fine.

He will get a horrid taste in his mouth