Many people don’t realize it, but acetaminophen (the active ingredient in Tylenol) is toxic for both dogs and cats. Acetaminophen is found not just in Tylenol, but also in many other over the counter pain relievers and cold/flu remedies.
This article by Pet Health Network gives a brief overview of the symptoms and treatment of acetaminophen poisoning — Acetaminophen toxicity in dogs and cats.
For a more technical discussion of acetaminophen poisoning, see the following article — Acetaminophen (Tylenol) Toxicity.
If you suspect your dog or cat MIGHT have eaten something containing acetaminophen, go to your veterinarian or nearest veterinary emergency facility immediately. Treatment success depends in large part on how soon it can be started after ingestion.