"Dr. Heathcliff and Mr. Spike" is the first segment of the fourth episode of the second season of the animated series based on the comic strip of the same name, Heathcliff (called Heathcliff and the Catillac Cats). The segment is inspired by the story of Dr. Jekyll and Mr. Hyde. It originally aired in 1986.
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.