Grapefruit — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
Small flesh pieces are safe; the peel, seeds and pith contain oils and compounds that can upset the stomach.
Short answer
A small lick of the pulp is generally safe, but grapefruit should be fed with care to cats. The real risks are the peel, seeds, and white pith, which contain oils and compounds that can irritate your cat's stomach and potentially cause vomiting or diarrhea. If your cat only had a tiny taste of the flesh, just watch for any signs of stomach upset over the next day. Offer fresh water, and contact your own vet if you notice any unusual behavior or digestive issues.