The flesh of citrus fruits like lemon, orange and lime is usually fine in small amounts, but the peel, seeds and white pith contain essential oils and psoralens that can irritate the digestive tract and cause vomiting or diarrhea. Large amounts can also cause psoralen-related photosensitive dermatitis. Citrus essential oils are especially risky for cats, so be cautious with diffusers and aroma oils too.