People with allergies to Equisetum arvense, related substances, or to nicotine should avoid horsetail. Rash has been reported in a patient taking horsetail who was known to be sensitive to nicotine.
Side Effects and Warnings
There are few scientific studies or reports of side effects with horsetail. It is more often used in Germany and Canada, where it is traditionally considered to be safe when taken in appropriate doses. Equisetum palustre (marsh horsetail) contains a poisonous ingredient and should be avoided. There are reports that some batches of Equisetum arvense (horsetail) have been contaminated with Equisetum palustre.
Large doses of horsetail may cause symptoms of nicotine overdose, including fever, cold hands and feet, abnormal heart rate, difficulty walking, muscle weakness, and weight loss. People who smoke or who use nicotine patches or nicotine gum should avoid horsetail. Reports from animal studies and one report of a nicotine-allergic person describe a rash occurring after the use of white horsetail. Other reports from use in animals describe nausea, increased frequency of bowel movements, increased urination, loss of the body's potassium stores, and muscle weakness. People with kidney disorders should avoid horsetail.
Studies in mice suggest that horsetail may change the activity of the kidneys, causing abnormal control of the amount of water and potassium release. Low potassium, which in theory may occur with horsetail, can have negative effects on the heart. Individuals who have heart rhythm disorders or who take digoxin should be cautious. Studies suggest that horsetail does not change blood pressure.
Horsetail contains an ingredient that destroys thiamine (vitamin B1), which could lead to deficiency with long-term use. This may cause permanent damage to the brain and nervous system, including confusion, difficulty walking, difficulties with vision and eye movement, and memory loss. People who have thiamine (vitamin B1) deficiency or poor nutrition should avoid horsetail, as it may affect levels of thiamine even more. Alcoholic or malnourished individuals are often thiamine deficient and this may be worsened by horsetail.
Avoid use in children due to anecdotal reports of poisonings while using horsetail stems as whistles.
Avoid use in patients taking antidiabetic agents, as a different horsetail species (Equisetum myriochaetum) has reportedly caused low blood sugar levels in type 2 diabetic patients. However, the effects of Equisetum arvense are unclear.
Avoid use in patients with gout or in those taking antigout agents, as horsetail has been shown to increase the formation of uric acid crystals in the urine.
Pregnancy and Breastfeeding
Horsetail is not recommended during pregnancy or breastfeeding, since little information is available about its safety. Its potential to cause thiamine (vitamin B1) depletion, low potassium, and nicotine-like effects are of particular concern. Many tinctures contain high levels of alcohol and should be avoided during pregnancy.
Most reported doses for horsetail are based on historical use or expert opinion. There is a lack of reliable studies available in humans that show horsetail to be effective or safe at any specific dose. Different doses of horsetail have been used, starting at 300-milligram capsules taken three times per day, up to 6 grams per day. A maximum of 6 cups of tea, containing 1.5 grams of dried stem in one cup of hot water, is a dose that has been used. A common dose for a tincture (1:1 in 25% alcohol) is 1 to 4 milliliters three times daily. To treat osteoporosis, a supplement containing 270 milligrams of Osteosil® calcium (a combination of horsetail and calcium) has been taken twice daily for one year. A wash prepared by mixing 10 teaspoons of horsetail in cold water and soaking for 10 to 12 hours has been applied on the skin.
There is not enough scientific information to recommend the use of horsetail in children. Poisonings have been reported in children using horsetail stems as whistles.
Interactions with Drugs
Some diuretic drugs ("water pills") can cause the body to lose water and potassium, for example loop diuretics like furosemide (Lasix®). The use of horsetail with certain diuretics may cause dehydration or further potassium deficiency. Some steroids and laxative drugs can also lower potassium levels and should not be combined with horsetail. Individuals with heart rhythm disorders who are treated with digoxin (Lanoxin®) or digitoxin may be especially sensitive to low potassium levels, and potassium levels should be monitored in such individuals.
Nicotine, a stimulant, may be found in horsetail. Because horsetail can stimulate the brain and nervous system, caution should be used when combining horsetail with stimulant drugs and nicotine.
Horsetail may interact with antigout agents, as horsetail has been shown to increase the formation of uric acid crystals in the urine.
Other horsetail species have caused low blood sugar and therefore horsetail may increase the effects of diabetes medications. However, clinical effects on diabetes therapies are unclear.
Horsetail may have additive effects when taken with agents that treat osteoporosis, as horsetail may increase bone density. However, horsetail's anti-osteoporosis effects are not well established.
Many tinctures contain high levels of alcohol and may cause nausea or vomiting when taken with metronidazole (Flagyl®) or disulfiram (Antabuse®).
Interactions with Herbs and Dietary Supplements
Increased urine production, dehydration, or electrolyte imbalances may theoretically occur when horsetail is used with herbs that may increase urination. Dehydration or low potassium levels also may theoretically occur if horsetail is used with laxatives. Horsetail may also interact with herbs or supplements taken for gout or osteoporosis, although supportive evidence is currently lacking.
In theory, low potassium levels caused by horsetail may be dangerous in people using herbs that have cardiac glycoside activity on the heart such as foxglove and oleander. Other potassium-depleting herbs, such as licorice, should also be avoided when taking horsetail.
Horsetail may interact with stimulants and herbs and supplements with similar properties such as ephedra and licorice.
Other horsetail species have caused low blood sugar and therefore horsetail may increase effects of diabetes medications. However, clinical effects on diabetes therapies are unclear.
Horsetail may break down thiamin and may cause thiamine deficiency. Horsetail may also have additive effects in patients taking antioxidants.