Horsetail is one of the oldest medicinal herbs–it even precedes dinosaurs on earth. People have been brewing tea and cooking with horsetail since ancient Greek and Roman times. Read on to learn about its modern-day uses and side effects.
What is Horsetail?
Horsetail (Equisetum) got its name due to its resemblance to the tail of a horse.
Out of 15 horsetail species, common horsetail (Equisetum Arvense) is best known for its health benefits. It is native to North America, Europe, the Middle East, and parts of Asia. Other horsetail species are recently gaining popularity around the world. Giant horsetail (Equisetum giganteum) is found only in Latin America .
However, there is insufficient evidence to rate the effectiveness of horsetail for most uses. Proper clinical studies are needed to determine the purported health benefits of horsetail. With this in mind, we’ll discuss the studies that have been published so far and point out the directions future research may take.
Additionally, horsetail supplements have not been approved by the FDA for medical use. In general, dietary supplements lack solid clinical research. Regulations set manufacturing standards for supplements but don’t guarantee that they’re safe or effective. Speak with your doctor before supplementing.
The aerial parts of the plant are used for their health benefits. Ancient Romans used horsetail as food, medicine, and animal feed. In fact, people eat horsetail as a salad in some parts of Europe [2, 3].
Horsetail was historically prepared as a juice, tea, or tincture for treating many diseases. It’s best known in folk medicine for treating swelling, weight loss, diabetes, bladder disease, kidney disease, arthritis, tuberculosis, and other infections .
Horsetail is being studied for bone, oral, hair, and nail health. This is because horsetail is actually the most abundant source of silica in the plant world [5, 6, 4, 7, 8, 9].
Horsetail ointment can be applied to heal wounds, stop bleeding, prevent infection, and reduce pain .
Components of Horsetail
Horsetail is rich in beneficial compounds that fight inflammation and infections. It also contains vitamins, minerals, and dietary fiber.
The exact chemical composition varies depending on the following [10, 11, 12]:
- Geographical origin
- Extraction process
- Processing method and storage
Horsetail contains many useful active ingredients:
- Phenolic compounds such as flavonoids and phenolic acids have antioxidant, anti-inflammatory, liver-protecting, antimicrobial, and antitumor effects. These compounds are extremely diverse inactivity and chemistry 
The main phenolic compounds are apigenin, luteolin, flavan-3-ol, kaempferol, isoquercitrin, quercetin, proanthocyanidins, tannins, caffeic acid, and other phenolic acids [13, 12, 14, 15, 1, 16].
- Silicon and silica boost collagen production, and strengthen hair, bones, teeth, and nails [17, 6, 18, 19, 20, 21]
- Kynurenic acid reduces inflammation and pain [22, 23]
- Styrylpyrones may protect against cancer [24, 25]
- Water-soluble vitamins including ascorbic acid (vitamin C), thiamine (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), pantothenic acid (vitamin B5), pyridoxine (vitamin B6), and folate 
- Fat-soluble vitamins, such as vitamins E and K 
- Trace minerals and other elements like potassium, sodium, calcium, magnesium, iron, zinc, and copper [3, 26]
- Volatile compounds from horsetail essential oils (thymol) have strong antimicrobial effects .
- Inositol 
- Choline 
- Chlorophyll (sometimes removed in extracts), which may prevent cancer, and carotenoids (provitamin A) [3, 28].
- Phytosterols, such as beta-sitosterol and campesterol .
- Dietary fiber 
It also may contain traces of alkaloids that can cause side effects [4, 31].
Mechanism of Action
The source of the medicinal benefits of horsetail come from its silica and antioxidant compounds. They act both alone and together to achieve unique beneficial effects .
Cell-based and animal studies suggest that horsetail may act by*:
- Decreasing inflammation:
- Boosting antioxidant defenses:
- Reducing free radicals, which damage healthy cells 
- Increasing antioxidant enzymes (SOD and glutathione) 
- Reducing fat peroxidation, which contributes to many chronic diseases [34, 40]
- Increasing PPAR alpha and PPAR gamma 
- Reducing 5α-reductase: An important enzyme for testosterone production and hair loss in men 
- Increasing the uptake of calcium, remineralizing bones and teeth, and regenerating tissues 
- Increasing the production of collagen, which strengthens and improves the elasticity of the skin, joints, and blood vessels 
- Reducing bleeding and improving wound healing 
- Stopping or slowing down the growth of bacteria, viruses, and yeast by disrupting their cell walls and energy production [27, 43]
*It’s unknown if horsetail acts by the same mechanism in humans. Additional research is needed.
Horsetail Risks and Safety
Thiaminase and other Alkaloids
Horsetail is generally safe, and short-term use is not associated with side effects. Horsetail does carry some risks and is not safe to use in some populations .
- Marsh horsetail (Equisetum palustre) causes poisoning in livestock. It contains highly toxic alkaloids and thiaminase. Thiaminase breaks down thiamine (vitamin B1) and causes serious symptoms of vitamin deficiency. Avoid marsh horsetail supplements, and buy horsetail from a reliable manufacturer [44, 45].
- Quality manufacturers will ensure horsetail supplements do not contain thiaminase (“thiaminase-free” products). But it’s still possible that the enzyme has not been completely removed. People with low thiamine should avoid horsetail or take special precaution .
Thiamine deficiency can be suspected in:
- Alcohol abuse
- Liver disease
- Some brain disorders
- Genetic disorders
- Avoid drinking alcohol on a regular basis when taking horsetail to reduce the risk of thiamine deficiency.
- Common horsetail can contain traces of nicotine. People using nicotine patches or nicotine chewing gum should avoid horsetail .
- Horsetail can reduce blood glucose levels. People with diabetes should regularly monitor their glucose levels if taking horsetail .
- Long-term use of oral horsetail supplements is not recommended, as no long-term safety studies exist.
- Horsetail may cause skin allergies. Rats fed large amounts of horsetail on a cholesterol-rich diet developed a skin allergy .
- One case of hepatitis was reported after a male took horsetail tea for a week .
- Pregnant and breastfeeding women, as well as children (age < 18 years) should not use horsetail due to lack of safety data.
- Horsetail increases the elimination of potassium due to its diuretic effect. This can cause low potassium levels. Horsetail should not be used in people with a risk of potassium deficiency or in people with heart arrhythmias .
- People with allergies to carrots and nicotine: Some people with an allergy to carrots or nicotine might also be allergic to horsetail and should avoid it.
Several prescription medications may interact with horsetail :
- Lithium: Horsetail could interfere with lithium elimination and cause a dangerous buildup
- HIV medications: Horsetail may cause worsening of HIV if taken with HIV drugs
- Diuretics (water pills)
- Digoxin (for heart failure)
- Phenytoin (anti-seizure)
- Anti-diabetes drugs
Additionally, horsetail contains chromium (0.0006%). It may increase the risk of chromium poisoning when taken with chromium supplements or chromium-containing herbs such as bilberry, brewer’s yeast, or cascara.
Consult your healthcare provider before taking horsetail or any other supplement if you are on prescription medications. Drug interactions can lead to serious adverse events.
Forms of Supplementation
Horsetail is usually used in the following forms:
- Fresh/dried herb
- Essential oil
- Cream, lotion, ointment, and powder for hair/skin use
Choosing a horsetail supplement depends on the sought-out health benefits. Standardized extracts are usually of higher quality and have a specified amount of the active ingredient. At least 0.3 % of total flavonoids should be present in the dried plant, standardized to isoquercetin .
Tea seems like the easiest and least expensive solution. But, water does not extract some active ingredients as well as other solvents (like alcohol). Tea will have the mildest effect and the lowest concentration of antioxidants .
- Extracts: 900 mg per day of the dried extract of common horsetail standardized to 0.026% total flavonoids was used for diuretic effects in healthy volunteers. The daily dose was split up to 300 mg 3 times in a day .
Otherwise, the dosage is based on traditional use. Most supplements contain 300 mg dried extract per capsule/tablet, and depending on the extract type, can usually be used up to 3 times daily [55*].
The daily dose for most alcohol extracts based on traditional use is 2 to 12 ml per day (depending on the extract type) [55*].
- Tea: 1 to 4 g of the dried herb can be used to make tea, to be taken 3 to 4 times daily over 2 to 4 weeks (based on traditional use) [55*].
- Ointment: A sterile 3% common horsetail ointment helped wound healing and reduced pain after surgery in one study .
*According to the European Union herbal monograph on horsetail prepared by the Committee on Herbal Medicinal Products.
- A combination of horsetail with L-lysine, L-proline, L-arginine, and vitamin C was used in one study to prevent and treat osteoporosis in aged female rats who don’t produce estrogen. It boosted collagen production, bone development, and strength. Osteoporosis is common in women after menopause, as estrogen production decreases .
- A combination of horsetail, soy isoflavones, vitamin D3, and lactoferrin improved osteoporosis in aged rats. It had no effect on younger rats .
- Nourkrin is a supplement for treating hair loss that contains horsetail, fish proteins, acerola cherry, silica, and D-biotin. It improved hair loss after 6 months in a study with 55 men suffering from hair loss .
- Viviscal is an oral supplement for treating hair loss containing silica from horsetail, fish extract, biotin, zinc, vitamin C, and iron. It increased hair growth, volume, and thickness in women with thinning hair after 3 months. Viviscal achieved similar results in a study with men experiencing hair loss [60, 61].
- Eviprostat used to treat enlargement of the prostate (benign prostatic hyperplasia – BHP), contains common horsetail, prince’s pine, aspen, small pasque flower, and purified wheat germ oil. Eviprostat has been used in Japan and Germany for over 40 years .
- Eviprostat also shows anti-inflammatory and antioxidant activity. Eviprostat reduced oxidative stress in 9 patients with BHP. In an open clinical study of 100 patients with BHP, it reduced symptoms and improved quality of life. Eviprostat improved prostate inflammation and pain in 50 patients with prostate inflammation [62, 63, 64, 65].
Kidney and Bladder Support
- A herbal combination with common horsetail, verbena, gromwell, dandelion, bearberry, greater burdock, and Silene saxifraga was used to treat kidney stones in rats .
- Herbensurina used to prevent treat kidney stones in rats contains horsetail, smooth rupturewort, couch grass, and elderberry .
- Urox is a supplement for bladder support that contains horsetail, varuna, and lindera plant extracts. It reduced overactive bladder and urinary incontinence in a study with 150 participants. Urox reduced symptoms of urgency, frequency, incontinence, and nighttime urination after 8 weeks .
- A Japanese herbal patent for reducing cavities and bad breath contains horsetail as one of the ingredients. The herbal combination may act by blocking an important enzyme. More studies would need to tease out the benefits of horsetail in the combination .
- Nail polish containing horsetail, hydroxypropyl-chitosan, and methylsulfonylmethane improved nail psoriasis on all fingernails after 6 months in a study of 87 patients with psoriasis. Psoriasis is a chronic inflammatory skin disease, and half of the people suffering from it will also experience nail damage .
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Many users said horsetail capsules helped them regrow thinning hair or strengthen hair and nails, usually in combination with other supplements and stress reduction.
One user reported eliminating kidney stones after drinking horsetail tea for a month instead of water.
Another user didn’t notice improvement after taking horsetail powder for 6 months to regenerate knee meniscus damage.
Some users experienced stomach burning and cramps after taking horsetail over 2 to 4 weeks for hair, skin, and nails. The burning went away once horsetail was discontinued. One user mentioned experiencing similar symptoms with other hair supplements with high silica content.
People traditionally used horsetail to reduce fluid buildup and inflammatory issues. Today, it’s mostly used as a source of silica, which is claimed to support bone, hair, and skin health.
Popular supplements for hair growth combine it with fish oil, biotin, other vitamins, and plant antioxidants.
Caution is advised since horsetail supplements can contain thiaminase, which is dangerous for people at risk of deficiency.
Horsetail is counterindicated during pregnancy, breastfeeding, in people with potassium deficiency or heart problems, and in people with allergies to carrots and nicotine.
It can also interact with medications, so be sure to consult your healthcare provider before supplementing.