Why a tablespoon hits cats harder than dogs
Even plain yogurt still carries some lactose, and cats lack the enzymes to break it down efficiently. For a 5 kg cat, that tablespoon is like a human drinking a full glass of milk—enough to overwhelm their system. The diarrhea you're seeing is the gut's way of flushing out what it can't digest, not an infection or toxin at work.
Watch for straining or mucus in the stool, which suggests irritation. The intestines should settle within a day if no more dairy is given. Withhold treats but not water—dehydration from fluid loss is the only real risk here.
Timing of lactose reactions in small bodies
Cats often show digestive upset within 4-6 hours of eating dairy, peaking around the 8-hour mark. Your cat's mild diarrhea at the 6-hour point fits this pattern exactly. The worst should be over by now unless they've licked more yogurt secretly.
If stools haven't firmed up by tomorrow evening, consider whether your cat got into other foods too. Persistent diarrhea beyond 24 hours warrants a vet visit to rule out coincidental parasites or illness.
How to support recovery without meds
Skip the next meal to let the gut rest, but don't fast longer than 12 hours for an adult cat. Offer bland cooked chicken (no skin or seasoning) in tiny amounts once stools start forming again. Probiotics aren't needed—their own gut flora will rebalance.
Litter box monitoring is your best tool. Note whether the diarrhea is truly watery or just soft, and check for blood streaks. Normal playfulness and drinking habits mean this is just a passing lactose overload.
When yogurt becomes a vet-level issue
Two scenarios would shift this from simple intolerance to urgent care: violent vomiting alongside the diarrhea, or complete refusal to drink water. Cats dehydrate fast, and yogurt alone shouldn't cause nausea.
For this tablespoon amount, lethargy or bloating would be unexpected. Those signs would point to an obstruction from another swallowed object, not the yogurt itself. Stick to observing stool changes for now.
The full verdict, portion guidance and serving notes are collected on the Plain yogurt food facts page.