Side Effects of Selected Herbs

By Shawn Messonnier, DVM

In my practice, I prefer using natural therapies instead of conventional medications whenever possible. Usually, these natural therapies are much safer than their conventional drug counterparts, and cause few if any side effects. While many think that 'natural' always equals 'safe,' this is certainly not true. Any therapy, be it a conventional drug or herb or nutrient, can have side effects.

Sometimes the use of an herb or supplement can have a side effect by itself, whereas other times the herb or supplement might interfere with the actions of a conventional medication or increase the chance of side effects when used with a certain medication. Side effects can be directly related to the supplement or due to contamination of the supplement during processing (this is especially true with herbal remedies). To decrease the chance of side effects from contamination, it is important to only use products from reputable manufacturers.

However, in most cases, when used correctly, natural therapies are less likely to have the side effects often encountered when conventional medications are used. For example, even short term use of NSAIDS can result in serious side effects, whereas natural joint supplements like glucosamine are generally used without harm to the patient. This article will review some of the most commonly prescribed supplements and their side effects.

Astragalus - The medicinal herb Astragalus membranaceous is safe and is used as an immune-boosting supplement; many other species of Astragalus are toxic. For immune system disorders (autoimmune diseases, diabetes) and disorders with diminished immune systems with low white blood cell counts (feline leukemia and immunodeficiency diseases), it may be wise to avoid this herb as Astragalus is used for immune stimulation. It is best used early in the course of the disease to stimulate the immune system.

Black Walnut - Black walnut is often used by pet owners as a natural deworming agent, especially to treat heartworm disease. While the history of the herb supports its use to treat parasites, there is no consistent proof of its use as a single agent to treat heartworm infection. This herb is usually considered too toxic to use without veterinary supervision. The tannins and alkaloids may lead to vomiting and diarrhea. Most conventional dewormers (and other herbal deworming preparations) are much safer.

Borage - Borage is often used for its fatty acid (EFA) content, for its ability to mildly strengthen the adrenal gland, and as a respiratory expectorant. Borage leaves contain small amounts of compounds (alkaloids) that can be toxic to the liver; therefore, borage should not be used in pets with liver disease or in pregnant animals. Use of large amounts of borage, or prolonged ingestion of large amounts should be avoided.

Bugleweed - Bugleweed is recommended by herbalists to help counteract the effects of mild hyperthyroidism, and also as a safer alternative to digitalis for the treatment of mild cardiovascular disorders. Considered safe but should not be used in pregnant or nursing animals as it can constrict blood vessels and may have hormonal properties. Do not use in pets with hypothyroidism.

Cayenne - Cayenne is well known in the human and animal literature for its topical use as an anti-inflammatory and analgesic agent, possibly by inhibiting substance P. Cayenne can be irritating to mucous membranes; avoid use in animals with sensitive skin. Animals with sensitive digestive or urinary system disorders, as well as pregnant animals, should not be given cayenne orally.

German Chamomile - Chamomile is well known for its sedative effects. Avoid in pregnant animals as it may cause abortion. Usually considered a safe herb, the rare pet may be allergic to chamomile.

Chaparral - Chaparral is reported to be an effective antimicrobial herb. However, ingestion of large amounts can lead to liver damage; avoid in pets with liver disease; potentially a very toxic herb and not usually recommended.

Red Clover - Red clover is used in many herbal cancer formulas due to its diuretic, blood cleansing, and anti-neoplastic effects. Red clover contains coumadin and should not be used in pets with blood clotting disorders. If fed in large amounts, the estrogenic components can be toxic. Do not use in pregnant animals. Red clover contains very small amounts of salicylic acid (aspirin), and care should be used in pets taking corticosteroids or non-steroidal medications and in cats which are sensitive to salicylic acid.

Coltsfoot - Coltsfoot is recommended for its expectorant and antispasmodic effects for patients with respiratory disease. The flowers (not the leaves and stems) contain small quantities of alkaloids that can cause liver damage or cancer if taken in large quantities. Use only as directed and for short periods of time. Do not use in pregnant animals or pets with liver disease.

Comfrey - Comfrey is recommended for its anti-inflammatory and lubricating properties. Comfrey contains small quantities of alkaloids that can cause liver damage or cancer if taken in large quantities. While the leaves (the most commonly used part of the herb) contain almost negligible amounts of alkaloids (the roots contain the most and should never be used), it is wise to use comfrey only as directed and for short periods of time or avoid it altogether. Many doctors consider it too toxic to use.

Echinacea - Echinacea is a well-known immune modulating supplement. For immune system disorders (autoimmune diseases, diabetes) and disorders with diminished immune systems with low white blood cell counts (feline leukemia and immunodeficiency diseases), it was recommended in the older literature to avoid this herb as echinacea is used for immune stimulation. However, there have been no clinical studies supporting this recommendation, and echinacea has been safely used in people with these disorders. The older literature also recommended not using the herb for longer than 4-8 weeks without giving the body a 'break,' but again this has not been substantiated clinically and it has in fact been used for longer periods of time without harm. Most veterinarians prefer to use echinacea early in the course of the disease at the first signs of infection to properly and fully modulate the immune system. Caution is warranted in diabetics as the condition may become unstable.

Ephedra - Ephedra has a long history of use in Traditional Chinese Medicine as an effective therapy for respiratory (especially asthmatic) disorders. While it has been reported that cats may exhibit idiosyncratic reactions, I have not had any side effects in cats treated with ephedra for upper respiratory disease. Ephedra, most commonly prescribed for pets with asthma or respiratory problems, can cause heart arrhythmias and high blood pressure. Use with great caution in all pets. It should be combined with other herbs to allow use of the lowest dose of ephedra possible. It should not be used when medications which have similar actions are used (MAO inhibitors, sympathomimetics) or in pets with hypertension, cardiac arrhythmias, anxiety, restlessness, glaucoma, cardiovascular disease, impaired cerebral circulation, prostatic adenoma with residual urine accumulation, pheochromocytoma, or hyperthyroidism.

Garlic - Garlic has been historically recommended for many uses, including the treatment of parasites, microbial infections, and in the treatment of cancer. Garlic in large amounts can cause Heinz body anemia in dogs and cats due to the presence of S-methyl cysteine sulfoxide and N-propyldisulfide. Do not use in pets with anemia. Garlic in high doses can prolong bleeding times. As a general guideline, 1 clove of garlic per 10 pounds of body weight for dogs (and 1/2 clove per cat) can usually be fed safely each day.

Ginkgo Biloba - Ginkgo is well known for its use in treating mild forms of cognitive disorder and intermittent claudication in people. Ginkgo has antithrombotic activity via its PAF inhibition. Caution should be used if ginkgo is given to patients taking anticoagulant or antithrombotic medications (aspirin, NSAIDS). It has been suggested that antiplatelet medications and herbs be stopped about 1 week prior to surgery. Rare reports of spontaneous bleeding (subdural hematomas, hyphema, subarachnoid hemorrhage) are reported in the human literature. No reports are noted in pets. Do not use in animals with blood clotting disorders. Do not use in pregnant animals.

Goldenseal - Do not use in pregnant animals or those with hypoglycemia (it can lower blood sugar). Goldenseal may cause hypertension and should not be used in pets with kidney failure or in cats with hyperthyroidism. Long term use can result in excess bile production and can cause vomiting, especially in cats. It may aggravate liver disease.

Gotu Kola - This herb is recommended to stimulate the healing of skin disorders when used topically and for internal use as a mild diuretic. It has also been recommended in the treatment of epilepsy and arthritis and as a mild sedative. Do not use in pregnant animals. Excessive doses may cause narcotic-like effects, interfere with hypoglycemic therapies, and may increase sensitivity to sun exposure. May interact with anxiolytic medications.

Kava kava - Kava has a long traditional history of being a good calming, sedative herb. Can be toxic to the liver in excess amounts and it should not be used in pets with liver disease. There have been recent reports of liver toxicity and death in depressed people treated with this herb. However, careful analysis of these reports revealed that these patients had preexisting liver disease, were taking drugs with potential hepatotoxicity, or were suffering from chronic alcoholism. The herb has a long history of safety but it is recommended to screen for liver disease before using the herb and to periodically monitor liver enzymes if the herb needs to be given for long-term use. Do not use in pregnant animals. May interact with anxiolytic medications.

Marshmallow - This herb is used for its soothing and lubricating properties in gastrointestinal and urinary disorders. Use with caution in hypoglycemic pets. May retard the intestinal absorption of drugs when given with medications.

Milk Thistle - Milk thistle is well-known for its treatment of liver disease. Do not use in pregnant animals. Long term use in normal animals may result in depressed liver function unless chronic liver disease is present. It is not recommended to use milk thistle to prevent liver disease.

Oregon Grape - Oregon grape is used for its antimicrobial properties. Do not use in pregnant animals. Do not use in animals with liver disease. Excessive dosages may deplete vitamin B complex levels.

Passionflower - This herb is used for its sedative effects. Do not use in pregnant animals. Excessive doses may cause sedation and potentiate the effects of drugs that are monoamine oxidase (MAO) medications.

Pennyroyal - While pennyroyal oil is an effective insecticide, due to potential severe toxicity and death pennyroyal oil is not recommended for use in pets.

Tea Tree - Tea tree oil is used topically for its antimicrobial effects. It is generally recommended not to use volatile oils in cats, or only do so with proper dilution and supervision. Small-breed dogs may also be sensitive to undiluted oil. Dilute the oil with vegetable or almond oil (at least 50:50). Test a small patch of skin prior to use as some pets may be sensitive.

Valerian - This herb is used for its sedative effects. Do not use it in pregnant animals. It can cause gastrointestinal upset in large doses. Do not use with barbiturates or benzodiazepines.

Wormwood - This is another traditional deworming herb. Unsafe for internal use in people. Do not use in pets with seizures, kidney disease, liver disease, or in pregnant animals. Safer herbs for deworming exist and wormwood should only be used with extreme caution.

St. John's Wort - This is used as a natural sedative. Some pets may develop sensitivity to sun exposure. May interact with other similar medications. Serotonin syndrome may occur if combined with SSRI medications.

Yucca - Yucca is used for its nutritional value and as an anti-inflammatory agent. In large doses (in excess of 15% yucca root) or for long periods of time, vomiting may occur. Do not use for more than 5 days at a time.

About the author:

Dr. Shawn Messonnier is the author of "The Arthritis Solution for Dogs", "The Allergy Solution for Dogs", "8 Weeks to a Healthy Dog", and the award-winning "The Natural Health Bible for Dogs & Cats". Visit For your weekly dose of holistic pet care, read Dr. Shawn's column, "The Holistic Pet", in your local paper, distributed by Knight Ridder.