Herbs are useful to treat many medical conditions in dogs, but almost any herb can be toxic in excessive amounts. Most herbs are safe to use, especially with the advice of your vet. Some herbs should be used with caution, and a few shouldn't be used at all.
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Alkaloids
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Borage, comfrey, coltsfoot and Oregon grape all contain alkaloids that can permanently damage the liver, leading to liver failure and death.
Anticoagulants
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Alfalfa, chamomile, gingko biloba, hawthorne berry and red clover are all anticoagulants, meaning they delay clotting time of the blood. If given in excessive quantities, these herbs could cause a dog to have blood that won't clot, which could be fatal.
Anise
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Anise is typically irritating to dogs and can cause an allergic reaction, sometimes severe, including anaphylactic shock that can lead to death.
Pennyroyal
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Pennyroyal is used to kill fleas in bedding and carpets, but can be toxic to dogs if ingested.
Lavender
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The most common form of lavender is lavender oil. Lavender is toxic to dogs' livers and kidneys and can be fatal.
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