Species safety: cats

Why cats are different

Source: Court MH, Greenblatt DJ. Pharmacogenetics 2000;10:355-69 (feline UGT1A6).

The cat liver lacks the enzyme UGT1A6, which binds many medicines to glucuronic acid so the body can excrete them. Every domestic cat carries a non-functional form of this gene. That is why paracetamol is toxic to cats at any dose, and why aspirin is eliminated much more slowly than in dogs. The same mechanism is used to explain the high sensitivity of cats to some pyrethroids, although that last point is still a hypothesis.

In the reference: Paracetamol (acetaminophen)

Permethrin

Source: Boland LA, Angles JM. J Feline Med Surg 2010 (42 cats, permethrin).

Source: Kelmer E et al. Israel J Vet Med 2020;75(3) (61 cats).

Source: VMD (United Kingdom), advice on permethrin, 2014.

• Concentrated dog spot-ons are toxic to cats. Spot-on solutions for dogs contain 45-65% permethrin. The most common cause of poisoning in cats is such a spot-on applied to a cat by mistake, or a cat in contact with a freshly treated dog.
• Signs: muscle tremors and twitching, hyperaesthesia, seizures, fever, hypersalivation, ataxia, dilated pupils. They usually start within a few hours, sometimes after 24-72 hours.
• Outcome: with timely treatment most cats survive; mortality in different studies ranges from 0 to 40%.
• Keep a treated dog away from cats for 72 hours.
• Some shampoos and spot-ons whose label also lists cats contain permethrin at a low concentration (roughly below 1%). Use such a product only as its label says, keeping to the kitten age limit.

In the reference: Permethrin

Other pyrethroids

Source: Seresto Foresto 1.25 g + 0.56 g, SPC (UPD 700000105524) ↗

Source: Scalibor Protectorband 1.0 g, SPC (UPD 600000061275) ↗

• Flumethrin — authorised for cats by the label (a collar, together with imidacloprid). So „all pyrethroids are toxic to cats" is not correct.
• Deltamethrin, cypermethrin, cyfluthrin, D-cyphenothrin — use only on the species written on the label. The dog deltamethrin collar is not authorised for cats.

In the reference: Flumethrin, Deltamethrin

Organophosphates and carbamates

Diazinon, chlorpyrifos, propoxur, carbaryl inhibit cholinesterase. The safety margin is narrow: signs are hypersalivation, vomiting, diarrhoea, tremor, constricted pupils. Do not use together with another cholinesterase inhibitor. Follow the label species and dose exactly.

In the reference: Propoxur

Amitraz

Use only on the species and at the concentration written on the label. Dog amitraz collars and spot-ons are not authorised for cats.

Medicines and substances that are toxic or dangerous in cats

From the reference's data: toxicity records and critical contraindications for cats, each with its source.

Toxic

• Benzocaine: Benzocaine on a cat's skin can cause methaemoglobinaemia within minutes.
  A cat developed respiratory distress, vomiting and collapse 15-20 minutes after benzocaine cream was put on its skin; intravenous methylene blue reversed it. It is an ester-type local anaesthetic in human pain and teething creams, gels and sprays: do not use benzocaine products on cats.

  Source: Wilkie & Kirby 1988
• Diclofenac: No threshold is established; cats are considered much more sensitive than dogs, including to cream licked off an owner's skin.
  Source: ASPCA APCC: topical creams
• Fluorouracil: 5-fluorouracil (5-FU) is avoided in cats because of neurotoxicity and death.
  The usual exposure is a human 5% skin cream: a chewed tube, or licking skin treated with it. In a study, 2 mg/kg a day by mouth caused brain lesions in cats. No safe dose for cats is established.

  Source: Saba et al. 2013
• Ibuprofen: Cats are about twice as sensitive as dogs (limited glucuronidation).
  The cat thresholds come from poison-centre guidance and case reports, not from studies.

  Source: ASPCA APCC: feline exposures
• Naproxen: Cat data are scarce; cats are thought to be more sensitive than dogs.
  Treat any ingestion as significant.

  Source: ASPCA APCC: feline exposures
• Paracetamol (acetaminophen): There is no safe dose for cats: they can hardly glucuronidate paracetamol.
  Signs are reported from 10 mg/kg (part of one tablet), and in one report deaths at that dose.

  Source: MSD Vet Manual 2024
• Permethrin: Dog products containing permethrin must never be used on cats.
  Almost every poisoning is a concentrated dog spot-on (40-65% permethrin) put on a cat by mistake; close contact with a recently treated dog can also be enough. If it happens: wash the cat with a mild shampoo and get it treated at once; with intensive treatment most cats recover. No safe skin dose for cats is established.

  Source: VMD SPC 2026
• Phenazopyridine: Phenazopyridine causes methaemoglobinaemia and Heinz-body haemolytic anaemia in cats; no safe dose is established.
  A cat given 100 mg three times a day for 4 days developed haemolysis and died; in normal cats 65 mg/kg a day for 3 days turned nearly half of the haemoglobin into methaemoglobin, and even 10-20 mg/kg a day raised Heinz bodies. It is a human urinary pain medicine, often in cystitis combination products: do not give it to cats.

  Source: Harvey & Kornick 1976
• Tea tree oil: Concentrated (100%) tea tree oil, usually put on the skin by the owner against fleas or skin problems, poisoned 106 cats in ten years of North American poison-centre data; young and small cats were most at risk of major illness.
  Do not put undiluted tea tree oil on a cat.

  Source: Khan et al. 2014
• Theobromine: No feline dose study exists; dog thresholds are applied (one review gives a theobromine LD50 of about 200 mg/kg).
  Cats rarely eat chocolate because they cannot taste sweetness.

  Source: Kovalkovičová et al. 2009

Overdose risk / use with caution

• Acetylsalicylic acid: Aspirin is used in cats only at very low doses spaced days apart, prescribed by a veterinarian (for example against blood clots).
  Cats eliminate it very slowly (half-life about 37.5 hours), so dosing it like a human medicine makes it accumulate: 325 mg twice daily has killed cats, and 80-120 mg/kg over 10-12 days causes poisoning. Human tablets are too strong to dose accurately for a cat. No single-dose threshold is established.

  Source: MSD Vet Manual 2024
• Deltamethrin: Do not use on cats.
  Source: A critical contraindication in the reference (product label)
• Enrofloxacin: In rare instances, use of this product in cats has been associated with Retinal Toxicity.
  Do not exceed 5 mg/kg of body weight per day in cats. from 20 mg/kg Mydriasis, retinal degeneration... loss of vision. Mydriasis may be an indication of impending or existing retinal changes.

  Source: A critical contraindication in the reference (product label)
• Ketoprofen: Ketoprofen is authorised for cats in the EU (veterinary injection and tablets, EU Union Product Database).
  Cats are sensitive to NSAIDs: in one study 2 mg/kg a day for 3 days caused stomach lesions in adult cats. Human tablets are too strong to dose accurately for a cat. No ingestion threshold is established.

  Source: Takata et al. 2012
• Meloxicam: USA (FDA label, Metacam injection): cats get one injection only, 0.3 mg/kg under the skin.
  The boxed warning links repeated meloxicam in cats with acute kidney failure and death; give no further meloxicam or other NSAID after it. The US oral suspension is labelled for dogs only.

  Source: A critical contraindication in the reference (product label)
• Methylene blue: Given by mouth for days, as in old urinary antiseptic tablets, methylene blue caused Heinz-body haemolytic anaemia in cats, sometimes fatal; the US FDA regards oral methylene blue products for cats as unsafe.
  As the antidote to methaemoglobinaemia one intravenous dose of 1.5 mg/kg reversed it in cats without anaemia; a second dose 4 hours later raised Heinz bodies.

  Source: FDA 21 CFR 500.27
• Metronidazole: Neurotoxicity is reported in cats on high doses or long courses: 58 mg/kg a day for 6 months, 111 then 222 mg/kg a day, and 73.5-147 mg/kg a day for about 40 days (that cat had brainstem necrosis).
  In two cats the signs went within days of stopping the drug. Keep to the label dose and duration.

  Source: Caylor & Cassimatis 2001
• Propylene glycol: Propylene glycol taken by mouth every day (semi-moist foods, oral solutions) raises Heinz bodies and shortens red-cell life in cats in a dose-dependent way, already at 6% of the diet; at 41% of the diet it caused anaemia.
  The US does not allow it in cat food. It is also a solvent in some medicines: this record concerns repeated oral intake.

  Source: Christopher et al. 1989

When the label names the species only in general

If the leaflet says only „animals" or „animals, birds" and gives no separate dose for dogs or cats, treat the dose for that species as not established by the label.

What registration means

In Georgia veterinary medicines are registered by the National Food Agency. There are two routes: national registration (composition, quality control, safety and efficacy data for each species) and recognition of a medicine already registered in another country. In the EU, the UK and the US a new medicine needs target-species safety and field efficacy studies, while a generic is registered on documents and bioequivalence. Registration means the medicine is authorised for the species and dose on its label; using it in another species is the veterinarian's responsibility.

What the labels mean

• Toxic This species has no veterinary use at a proper dose.
• Not on the label for this species The label does not cover this species.
• Use with caution Allowed by the label, but the safety margin is narrow; follow the dose and age exactly.
• Overdose risk Used at a proper dose; an overdose is dangerous.

Sources

• Court MH, Greenblatt DJ. Pharmacogenetics 2000;10:355-69 (feline UGT1A6).
• Boland LA, Angles JM. J Feline Med Surg 2010 (42 cats, permethrin).
• Kelmer E et al. Israel J Vet Med 2020;75(3) (61 cats).
• VMD (United Kingdom), advice on permethrin, 2014.
• Scalibor Protectorband 1.0 g, SPC (UPD 600000061275) · medicines.health.europa.eu ↗
• Seresto Foresto 1.25 g + 0.56 g, SPC (UPD 700000105524) · medicines.health.europa.eu ↗
• Labels/SPC: EMA (Seresto, Scalibor), the Georgian register of veterinary medicines.
• Code on Food/Feed Safety, Veterinary and Plant Protection (matsne.gov.ge).
• The list: the reference's toxicity records and contraindications, each with its own source behind (i).