You can give your cat benadryl but you can only give them 1 mg of benadryl per pound of cat.
So if your cat weighs around 10 pounds you can give the cat 1/2 of a 25 mg benadryl tablet.
The cat can get very sick if you overdose the cat with the benadryl so make sure you're giving your cat the right dose of the benadryl.