Love the book, I love the way that Edgar talks to the dogs with his mind first and then his hands. He is emotionally much more like the dogs than he is people. I know what it feels like to "think" like the dogs and be one with them.
Here's what Dr. Gaynor had to say: "Acetaminophen (Tylenol) is well tolerated in dogs at appropriate doses. It has been shown to induce liver toxicity at high doses or if administered for long periods of time. This is the reason that many dog owners and veterinarians are scared of Tylenol use. Assuming that the dog in the story weighs about 100 pounds (45 kg) and the Tylenol is regular strength (325 mg/caspule), the dog in the book would have received 625 mg—about 14 mg of Tylenol per kilogram of body weight. This is within the currently accepted dose of 10 to 15 mg /kg orally two to three times daily for five days at a time.
This is very different in cats, which develop toxicity because of their inability to excrete the metabolites, which are the byproducts of liver metabolism."
The upshot of this is twofold. Most importantly, cats should never, under any circumstances, be given Tylenol. But Tylenol can and has been used with dogs. The two-tablet dosage described in the book would not have harmed Tinder, both because it is within range for Tinder's body weight and because it was a one-time event.