Why?
Lemon is the most acidic of the common citrus fruits, and its peel is concentrated in essential oils such as limonene, plus psoralens. Cats metabolize these compounds poorly, and they can cause vomiting, diarrhea, lethargy and skin sensitivity to sunlight. Luckily, the smell alone usually sends cats running.
How much?
No amount is recommended: no slices, no juice, no zest, no lemony fish. Also keep lemon-scented cleaning products and lemon essential oil out of reach.
The risks
- Vomiting and diarrhea
- Mouth irritation and drooling
- Lethargy, weakness, wobbliness
- Skin irritation or sensitivity to sunlight
Your cat ate some: what now?
If your cat only licked a little juice, mild irritation is the most likely outcome: watch for 24 to 48 hours. If your cat chewed peel or a whole piece, or is vomiting, drooling or seems weak, call your vet. Don't try to make your cat vomit yourself.
Better options
If you want to share your fish, leave it plain: that's how cats like it best. For something to chew on, try cat grass.
Common questions
Is lemon essential oil safe around cats?
No, and it's far riskier than the fruit: it's a concentrate of the very compounds cats process poorly. Don't diffuse it near your cat, never apply it to their fur, and call your vet after any ingestion or skin contact.
What about limes?
Same rule: limes are citrus fruits with the same oils and the same acidity. Avoid them too.
This information is general and does not replace advice from a veterinarian. If your cat has swallowed something toxic, call your vet or an animal poison control center right away, without waiting for symptoms.