Are Herbs Poisonous to Cats?

Some herbs are poisonous to cats. This may seem contradictory in that plants are considered “natural” and therefore cannot be harmful. However, herbs can trigger an allergic reaction or interfere with or increase possible side effects from medications already being taken by a cat. It is therefore important to consult with a veterinarian, preferably the one treating the cat, before administering any herbs to any pet. What may be good for human consumption may not be to a pet and so caution and investigation is necessary before any administration of any herbs.

Some Herbs and Their Effects

Some herbs can be dangerous or even toxic to cats in any dosage.

Though pennyroyal is an effective insecticide, it is very toxic to both dogs and cats and should not be used to treat any symptoms.

  • Since comfrey contains alkaloids, it can cause cancer or liver damage if taken in large amounts.
  • Though Ma Huang or Ephedra can treat asthma or respiratory problems in animals, it is not without risk and should be used with great caution. It can cause high blood pressure and heart arrhythmias (irregular heartbeat) in animals and possible idiosyncratic reactions in cats.
  • In small amounts, garlic can safely be used to treat parasites (fleas and worms), cancer and microbial infections. However, garlic can cause Heinz body anemia in pets when used in large amounts.
  • Undiluted tea tree oil is extremely toxic to cats. Even diluted tea tree oil should be tested for sensitivity on a small patch of skin before use on any animal.
  • Salicylates found in white willow bark are very toxic to cats and is also a blood thinner.
  • Though wormwood is a traditional deworming herb, it should be used with extreme caution and under the advise of a veterinarian.

Possible Herb Side Effects

Different cats may react differently to various herbs. Some reactions can be allergic reactions while other reactions can be dangerous or even fatal.

  • Sneezing
  • Swelling – this can be especially dangerous if the swelling affects breathing including swelling of the throat and/or nasal passages
  • Runny eyes
  • Runny nose
  • Itching – this can potentially be dangerous if the itching produces ulcers on the skin brought on by the constant scratching by the cat
  • Diarrhea – this can be potentially dangerous if the cat becomes dehydrated
  • Vomiting – this can be potentially dangerous since the cat can become dehydrated through the loss of fluids as well as malnourished since food will not stay down

Herbs and Interference with Conventional Medications

Some herbs interfere with conventional medications used in treating various medical conditions, such as infections or cancer. A veterinarian should always be consulted before administering any herb while a cat is taking other prescribed medications.

Such medications may include (but this is not an exhaustive list):

  • Antibiotics
  • Anti-inflammatory drugs (Rimadyl)
  • Aspirin
  • Cardiac Drugs
  • Central Nervous Systems Drugs (Phenobarbital)
  • Chemotherapy agents (cancer drugs)
  • Diabetic or hypoglycemic drugs (Insulin)
  • Diuretics (Furosemide, Diazide)
  • Hormones (Thyroxine)
  • Steroids


Various herbs can cause blood, liver, kidney or other potential deadly reactions. Therefore, a veterinarian should always be consulted first before administering any herb to a cat. Allergies can also be potentially hazardous to cats depending upon the severity of the allergic reaction.