When cats eat grapes, it may not seem like a major problem, but as with any human food, it is important to know whether there are any potential side effects or risks to your feline friend.
Can Cats Eat Grapes?
If a rebellious grape rolls off the table and your cat takes a nibble, there is no immediate danger, but this should not be a regular habit. Grapes and raisins may seem like an easy and delectable treat for your cat, but these small fruits do pose some health risks, and their nutritional content isn’t essential to cats’ health. The regular cat food that your kitten pal regularly eats has more than enough of the nutrients they need, so provided they eat regularly, there is very little need to add more food to their intake. The occasional treat or snack of human food you give your cat should be exactly that – occasional – and not seen as a regular dietary supplement. 
There are plenty of other small and non-controversial fruits and treats you can give to your pet besides grapes, so it is best to avoid them.
Are Grapes Poisonous to Cats?
Cats are not known to suffer side effects from grapes. But grapes do pose a very serious threat to their canine companions. Eating even a few grapes can cause acute kidney failure in dogs even though the exact toxin is not known. You can read Can Dogs Eat Grapes And Raisins for more information. There has been anecdotal evidence that a similar situation can occur in cats if they eat grapes, but there is very little proof to back up that claim. Even so, the consensus is not to introduce grapes to your cats’ diet intentionally.
Side Effects of Grapes for Cats
If your cat does eat a few grapes that roll off the counter, there is a low risk of side effects, but if they do appear, they will most likely come in the form of gastrointestinal distress, lethargy, and vomiting. If your cat suddenly loses their appetite, begins acting strangely, or seems to be depleted of energy, they could be experiencing a toxic reaction to the grapes. Most symptoms will appear within the first 12 hours, but keep a close eye on your cat for the next 24 hours just to be safe.