S2·E7 Dr. Heathcliff and Mr. Spike

Grandpa accidentally feeds Heathcliff some Super doggy Vitamins. Gradually, Heathcliff undergoes a startling transformation which includes barking at trucks and passing up fish in favor of bones. Worst of all, he finds himself feeling friendly toward Spike. Heathcliff eventually sets things straight by taking a massive dose of cat vitamins, with Iggy's help.