Why diarrhoea means kidney stress has already started
Diarrhoea in this case isn't just gastrointestinal irritation—it's one of the first signals that grape toxins are affecting your cat's kidneys. The 6 kg body weight doesn't provide much buffer against the concentrated toxins in just two grapes. Kidney cells are already struggling to filter out the harmful compounds, which is why waste products are exiting through the intestines instead.
Older cats like yours have less kidney reserve to begin with, so damage progresses faster. The diarrhoea means toxins are circulating right now—this isn't something that will pass on its own. You might not see vomiting yet because cats often mask nausea until it's severe.
How two grapes hit harder than you'd expect
People assume a couple of grapes are harmless because they're small, but feline kidneys react unpredictably to even tiny amounts. Unlike dogs where we sometimes see dose patterns, cats can crash from minimal exposure. Your cat ate roughly 0.3 grams of grape per kilogram of body weight—an amount that's triggered kidney failure in similar cases.
The grapes' sugar content isn't the concern here—it's the still-unidentified toxin that disrupts kidney function at a cellular level. Cooking or fermenting (like in wine) doesn't destroy it, and neither does the grape being seedless or peeled.
What happens in the next 6-12 hours without treatment
If untreated, the diarrhoea will likely worsen as kidney function declines, possibly turning bloody as intestinal lining becomes irritated. More concerning is what you won't see—the kidneys quietly failing to produce urine. By the time your cat stops using the litter box or vomits, up to 75% of kidney function could be gone.
Time matters because the grape toxin works fast—the first 18 hours post-ingestion are critical for intervention. IV fluids at the vet can help flush the toxin before it causes permanent damage, but they need to be started while the kidneys are still somewhat functional.
Why you can't watch and wait with grape exposure
Some toxins let you monitor for worsening signs, but grapes are different—by the time obvious symptoms appear, the window for effective treatment is closing. Your cat's current diarrhoea is essentially an early warning siren that many owners miss until their pet stops urinating entirely.
Blood tests now could show elevated kidney values before your cat acts sick. Don't assume that because she's still moving around or eating a little that the danger has passed. Kidney damage from grapes often becomes irreversible within 24 hours without aggressive treatment.
The full verdict, portion guidance and serving notes are collected on the Grapes food facts page.