Horsetail refers to various plant species of the Equisetum genus. It may help reduce fluid retention, but might cause vitamin B1 deficiency when used long-term.

The chemicals in horsetail might have antioxidant and anti-inflammatory effects. They might also work like “water pills” (diuretics) and increase urination.

People use horsetail for fluid retention, urinary tract infections (UTIs), osteoporosis, loss of bladder control, and many other conditions, but there is no good scientific evidence to support these uses.

There is interest in using horsetail for a number of purposes, but there isn’t enough reliable information to say whether it might be helpful.

When taken by mouth: It is possibly unsafe to take horsetail products that contain thiaminase. Thiaminase breaks down the vitamin thiamine. This could lead to thiamine deficiency. Some horsetail products are labeled “thiaminase-free,” but there isn’t enough reliable information to know if these products are safe or what the side effects might be.

When applied to the skin: There isn’t enough reliable information to know if horsetail is safe or what the side effects might be.

Special precautions & warnings:

Pregnancy and breast-feeding: There isn’t enough reliable information to know if horsetail is safe to use when pregnant or breast-feeding. Stay on the safe side and avoid use.

Alcohol use disorder: People with this condition are generally also thiamine deficient. Taking horsetail might make thiamine deficiency worse.

Allergies to carrots and nicotine: Some people with allergy to carrot might also have allergy to horsetail. Horsetail also contains small amounts of nicotine. People with nicotine allergy might have an allergic reaction to horsetail.

Low potassium levels (hypokalemia): Horsetail might lower potassium levels in the blood. Until more is known, use horsetail with caution if you are at risk for potassium deficiency.

Low thiamine levels (thiamine deficiency): Taking horsetail might make thiamine deficiency worse.

Moderate

Be cautious with this combination.

Efavirenz (Sustiva)

Efavirenz is a drug used to treat HIV. Taking horsetail with efavirenz might reduce the effects of efavirenz. Talk with your healthcare provider before using horsetail if you are taking efavirenz.

Lithium

Taking horsetail might decrease how well the body gets rid of lithium. This could increase how much lithium is in the body and result in serious side effects. Talk with your healthcare provider before using this product if you are taking lithium. Your lithium dose might need to be changed.

Medications for diabetes (Antidiabetes drugs)

Horsetail might lower blood sugar levels. Taking horsetail along with diabetes medications might cause blood sugar to drop too low. Monitor your blood sugar closely.

Medications for HIV/AIDS (Nucleoside reverse transcriptase inhibitors (NRTIs))

Taking horsetail with NRTIs might reduce the effects of these drugs. Talk with your healthcare provider before using horsetail if you are taking an NRTI.

Water pills (Diuretic drugs)

Horsetail can decrease potassium levels. “Water pills” can also decrease potassium levels. Taking horsetail along with “water pills” might make potassium levels drop too low.

Betel nut

Horsetail and betel nut both reduce the amount of thiamine available for the body to use. Using these herbs together might increase the risk for thiamine deficiency.

Chromium-containing herbs and supplements

Horsetail contains chromium. Taking it with other supplements that contain chromium can increase the risk of chromium poisoning. Examples of supplements that contain chromium include bilberry, brewer’s yeast, and cascara sagrada.

Herbs and supplements that might lower blood sugar

Horsetail might lower blood sugar. Taking it with other supplements with similar effects might lower blood sugar too much. Examples of supplements with this effect include aloe, bitter melon, cassia cinnamon, chromium, and prickly pear cactus.

Silicon

Horsetail contains large amounts of silicon. Taking horsetail along with silicon supplements might increase the risk of adverse effects from silicon.

Thiamine

Crude horsetail contains a chemical that breaks down thiamine. Taking horsetail might cause thiamine deficiency.

There are no known interactions with foods.

There isn’t enough reliable information to know what an appropriate dose of horsetail might be. Keep in mind that natural products are not always necessarily safe and dosages can be important. Be sure to follow relevant directions on product labels and consult a healthcare professional before using.

Alligator Cane, Asprêle, Bottle Brush, Cavalinha, Coda Cavallina, Cola de Caballo, Common Horsetail, Corn Horsetail, Dutch Rushes, Equiseti Herba, Equisetum, Equisetum arvense, Equisetum giganteum, Equisetum myriochaetum, Equisetum hyemale, Equisetum telmateia, Field Horsetail, Fleshy Grass-Tail, Giant Horsetail, Great Horsetail, Herba Equiseti, Herbe à Récurer, Horse Herb, Horsetail Grass, Horsetail Rush, Horse Willow, Paddock-Pipes, Pewterwort, Prele, Prêle, Prêle Commune, Prêle des Champs, Puzzlegrass, Sandpaper Plant, Scouring Rush, Souring Rush, Shave Grass, Shavegrass, Snake Corn, Snake Grass, Spring Horsetail, Swamp Horsetail, Toadpipe.

To learn more about how this article was written, please see the Natural Medicines Comprehensive Database methodology.

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