Cats can eat vitamin C. However, they should not be given large amounts, as vitamin C is quite irritating to a cat's oral mucosa and may cause a decrease in appetite after excessive intake. It's also important to note that human vitamin C should not be given to cats, mainly because the vitamin C requirements for cats differ from those for humans, and the dosage cannot be accurately controlled. Excessive intake can also be harmful to a cat's health. 
When do cats need vitamin C supplements?
1. Cats need vitamin C supplements when they are injured, as sufficient vitamin C can promote wound healing.
2. Kittens need vitamin C supplementation. Kittens grow quickly and are prone to vitamin C deficiency, so it is generally necessary to give kittens extra vitamin C supplementation.
3. Sick cats need vitamin C supplements. Sick cats have increased vitamin C requirements, but their body's ability to synthesize it is declining, so supplementation is necessary at this time.
