Generally, a small amount of lemon is sufficient to give your cat gastrointestinal symptoms The lemon tree, as well as the fruit, contains these poisons, so your cat (and other pets) shouldn’t be given free access to it. 

Think about how your cat responds to the smell of anything citrus. It may pull back and run to another area of your home as cats find any citrusy scent offensive. All citrus fruits (grapefruit, oranges, limes and lemons) are mildly toxic to cats. Consider every part of the fruit, from the seeds to the fruit and skin, to be toxic  for your feline.

The substances found in lemons (Citrus limon) is toxic to your cat, even though you and your family can safely ingest lemons.  Toxic compounds in the lemon include linalool and limonene, along with psoralens. The last compound is phototoxic, meaning it can cause your cat to suffer skin burns after exposure to sunlight.

After eating any part of a lemon, your cat will develop these symptoms:

Three compounds in a lemon, limonene, linalool and psoralen, are toxic, if not deadly for your cat. 

Limonene is a terpene that leads to the citrus scent of lemons. D-limonene has been used in dog shampoos and fragrances. The small amount present in dog products is safe for most sizes of dogs. For cats, it can prove lethal. Limonene is also used in flavoring compounds, cosmetic products, and cleaning products. Keep all of these away from your feline.

Linalool also gives the lemon its citrusy scent. It’s used as an insecticide in soaps and as a fragrant product in lotions. Linalool is also used as an insecticide itself.

Psoralen leads to photosensitivity issues for cats. It’s used as a treatment for certain skin disorders.

Luckily, cats don't tend to eat lemon as they dislike the sour taste. A single inquisitive lick will not do your cat any harm. If your cat eats lemon and you catch it, get it to the vet right away. Take the lemon or a part of the tree with you for testing to help your vet makes a diagnosis. 

Expect the vet to ask you several questions and give your cat a complete physical, including a urinalysis and blood work. The blood chemistry profile and complete blood count help your vet to rule out underlying conditions and determine better what toxins are affecting your cat. Your vet may also examine your cat’s stool and vomit specimens to identify the source of toxins. The potential for your cat’s symptoms to worsen increases with the amount of lemon or lemon tree your cat ate.

Your cat may also undergo neurological testing, which allows the vet to witness assess coordination and reflexes.

Once your vet knows what is causing your cat’s symptoms, she can determine the most appropriate treatment. Because the cat could breathe the essential oil of a lemon into its lungs, inducing vomiting isn’t an option. Instead, the vet will wash your cat’s stomach out (gastric lavage) to remove as much of the lemon and toxins from its digestive system. In addition, the vet will deliver activated charcoal to stop the absorption of any of the toxic compounds into its bloodstream.

If you found your cat eating a lemon, even though they find citrus scents to be so offensive, wash your cat’s fur and skin with a mild soap and clean, warm water.

Beyond that, the treatments your vet provides are supportive, including IV fluids that rehydrate your cat and adjust any electrolyte and blood glucose imbalances your cat may be experiencing. Your cat may receive supplemental oxygen and anti-seizure medications if its tremors are becoming severe.

Your cat should make a good recovery from its lemon poisoning, if you obtained quick veterinary care.  The poisoning symptoms are short-lived. Don’t take your cat’s poisoning lightly; if it eats any of the essential oils found in lemons, its prognosis may not be as good.

If your cat is an indoor-outdoor cat, keep it inside for about 48 hours after receiving treatment for phototoxicity. 

Your vet will have you bring your cat in so she can regularly monitor its blood chemistry levels. She is looking at how your cat’s liver and kidneys are functioning.

Before you bring your cat home, place all citrus products in a cabinet or inside the refrigerator. Citrus-scented products containing any of the known toxins should be kept where your cat can’t get to them. Read labels carefully when you buy cat care products or sprays meant to deter them from furniture or walls inside your home.