Quick answer: Drooling in cats usually indicates dental pain, nausea, or exposure to something toxic or irritating — it's not normal behavior and almost always has a specific cause.
Severe dental disease, tooth resorption, or mouth ulcers cause pain and inflammation that leads to drooling. If the drool has a bad smell, or your cat is eating less or chewing on one side, dental disease is a strong possibility. Over 70% of cats over 3 have some form of dental disease.
Cats drool when nauseous — from motion sickness, eating something that disagrees with them, or exposure to toxic plants or household products. If drooling started suddenly and your cat seems unwell or lethargic, contact a vet promptly. Many common houseplants are toxic to cats.
Some cats drool when deeply relaxed and purring — this typically starts in kittenhood and is completely benign. If your cat only drools while kneading in your lap with no other symptoms, it's likely a comfort response with no medical significance.
Occasional happy drooling is normal for some cats. Sudden or frequent drooling, especially with other symptoms, warrants a vet visit — don't wait to see if it resolves on its own.