Why the pit matters more than the fruit
Since your cat ate a whole plum but you're certain the pit was removed, the main risk is already avoided. Plum pits contain cyanide compounds, which are dangerous even in small amounts. The flesh itself is mostly water and sugar, which might cause mild digestive upset but isn't inherently toxic.
Cats often react to new foods with vomiting or loose stools simply because their systems aren't used to it. If your cat shows these signs within 24 hours, withhold food for a few hours before offering a bland meal. Persistent symptoms beyond a day warrant a vet check.
What to expect in the next 12 hours
Most cats process small amounts of fruit without issue, but you might notice increased thirst or slightly softer stools. These are normal reactions to the sudden sugar intake. The real concern would be signs of obstruction—if your cat starts straining in the litter box or acting uncomfortable, that's an emergency.
Watch for unusual lethargy or refusal to eat, which could indicate a more serious reaction. While rare, some cats have sensitivities to fruits that cause excessive gas or abdominal pain. If your normally active cat is hiding or vocalizing in discomfort, don't wait to seek advice.
When plum flesh becomes a problem
A single plum's worth of flesh is unlikely to overload your cat's system, but the high fiber content could cause temporary digestive slowdown. You might find undigested plum skin in the litter box—this isn't harmful, just evidence their body handled it imperfectly.
The sugar concentration becomes problematic only with repeated exposure. Cats lack the enzymes to efficiently process plant sugars, so frequent fruit snacks could lead to weight gain or long-term digestive irritation. This one-time event isn't cause for alarm unless preexisting conditions like diabetes are present.
Why this differs from dogs or human cases
Unlike dogs, cats rarely show interest in sweet foods unless exceptionally curious. Their smaller body size makes them more sensitive to any dietary change, but also means they're less likely to overconsume. Your cat's natural caution probably limited the intake to just this one plum.
Human food safety standards don't apply here—what's a healthy snack for you could be disruptive for your cat. The lack of pit removes the cyanide risk, but the fruit's acidity might still cause mouth irritation if your cat licked the flesh extensively. Check for excessive drooling or pawing at the mouth.
The full verdict, portion guidance and serving notes are collected on the Plums (pitted) food facts page.