What Is Horsetail?

Horsetail (Equisetum arvense or E. arvense) is a perennial herb in the Equisetaceae family of plants. Perennial means a plant returns every year and usually lives at least two years. Considered a weed, the horsetail plant is invasive and can spread through an area of land quickly, thus making it hard to get rid of once it is in your garden. 

The horsetail plant is like a fern. It has hollow, pointed stems and scaly leaves. Horsetail grows to be about 1 foot tall.

Typically the green fern-like part of the plant (i.e., the aboveground part) is used for medicinal purposes. 

People have been using horsetail since ancient Greek and Roman times. The plant has been used as a medicinal herb to treat weak and brittle bones (osteoporosis), the lung disease tuberculosis (TB), and kidney problems. Some have claimed that horsetail can help relieve fluid retention (edema), stop bleeding, and heal wounds.

Certain chemicals in horsetail are thought to lower inflammation. They might also help the body make more substances that boost the immune system (antioxidants). Horsetail also has silica and silicon in it. These minerals work together to strengthen hair and nails. They also help the body make healthy bone tissue.

While it has been used for a long time, horsetail's safety and its efficacy have not undergone enough rigorous research. This article discusses the potential uses of horsetail, its side effects, and precautions.

Uses of Horsetail

Supplement use is individualized and should be vetted by a healthcare professional, such as a registered dietitian, pharmacist, or healthcare provider. No supplement is intended to treat, cure, or prevent disease.

Horsetail has been studied in treating osteoporosis, rheumatoid arthritis, high blood pressure, high blood sugar, and hair growth.

Promotes Bone Formation

Osteoporosis is a condition that causes softening or thinning of the bone tissue, which can weaken the bones and cause them to break easily. It may occur in people going through menopause due to decreased estrogen levels. Fast bone loss occurs in about 25% of postmenopausal women.

Although there is a lack of human clinical study on the effect of horsetail on osteoporosis, one animal study found that the addition of E. arvense to the formulated mixture of basic bone mineralizing nutrients containing L-lysine, L-proline, L-arginine, and L-ascorbic acid increases bone formation in rats with ovaries removed, a condition that mimics menopause. However, it’s challenging to know the effect of horsetail alone.

Furthermore, in an experimental study done in rats, the mandibular (the bone of the lower jaw) bone mineral density in the 120 milligrams per kilogram (mg/kg) E. arvense group was higher than the control group as seen through radiography (a type of X-ray).

It’s important to highlight that the two studies above were conducted in rats. Not enough is known about the effects it will have in humans. More high-quality studies are needed with humans before further conclusions about the effectiveness of horsetail for osteoporosis may be made. 

Decreases Inflammation

A randomized controlled trial of 60 people with rheumatoid arthritis showed that the experimental group (horsetail mixture plus methotrexate and diclofenac sodium) was superior to the control group (methotrexate plus diclofenac sodium). Moreover, the horsetail mixture has decreased TNF-α (inflammatory marker) levels and increased IL-10 levels (anti-inflammatory marker).

The anti-inflammatory effect of horsetail has also been shown in an in vitro (test-tube) study on human primary lymphocytes (a type of white blood cell).

Lowers Blood Pressure

In a double-blind, randomized clinical trial of 58 people with stage 1 hypertension, the E. arvense group demonstrated a significant decrease in systolic (the top number) and diastolic (the bottom number) blood pressure comparable to Microzide (hydrochlorothiazide), a diuretic used to treat high blood pressure.

Further studies, including those with more participants, are warranted to determine the clinical potency of horsetail in high blood pressure.

Lowers Blood Sugar

Despite the lack of human clinical study on horsetail's effect on blood sugar, a study done in drug-induced diabetic rats showed that horsetail extract had a beneficial effect on blood sugar and insulin resistance (when cells stop responding to insulin and can no longer easily take up glucose from your blood). However, further investigation is needed to determine the exact mechanisms behind the effects.

Of note, as the study was conducted in rats, it is unclear if the results can be applied to humans with diabetes.

Promotes Hair Growth

Horsetail contains silicon, and it is suggested that hair strands with higher silicon content tend to have a lower falling rate and higher brightness. Additionally, silicon is essential for optimal collagen synthesis, improving skin strength and elasticity.

Horsetail has been used for other conditions, some of which lack clinical data demonstrating the efficacy of horsetail for these uses, include the following:

• Bleeding 
• Gout (a type of arthritis)
• Frostbite 
• Heavy menstrual periods
• Inability to control the bladder (incontinence)
• Kidney or bladder stones
• Osteoarthritis: Horsetail contains kynurenic acid, which has an anti-inflammatory effect. However, the level of kynurenic acid was lower in human subjects with rheumatoid arthritis than those with osteoarthritis.
• Urinary tract infections (UTI): A lab study suggested that E. arvense extract had antibacterial activity. However, the results need to be confirmed with clinical trials before recommending horsetail to prevent and treat UTIs.
• Wounds: According to a randomized placebo-controlled trial, 3% horsetail ointment applied for 10 days promoted wound healing in 108 people who had surgery to cause childbirth.
  

What Are the Side Effects of Horsetail?

Despite its potential uses, horsetail may have side effects. These side effects may be mild or severe.

Common Side Effects

• Increased urination: A double-blind, randomized clinical trial of 36 healthy male volunteers has shown that E. arvense extract produced a diuretic effect (fluid-flushing effect) comparable to the diuretic (water pill) Microzide (hydrochlorothiazide).
• Vitamin B1 (thiamine) deficiency: Since horsetail contains thiaminase, an enzyme that breaks down thiamine, taking it for a prolonged period may increase your risk of thiamine deficiency.
  

Severe Side Effects

Severe side effects may occur when too much horsetail is taken or when horsetail is taken with certain prescription drugs that can worsen such side effects.

Seek medical attention if you experience palpitations.

Avoid horsetail if you have an allergy to it or members of the Equisetaceae family. Stop taking horsetail immediately if you experience any signs of allergic reaction.

Precautions

Horsetail has several known precautions. Exercise caution if any of the situations listed below apply to you. 

• Alcohol use disorders: Chronic alcohol use can cause vitamin B1 (thiamine) deficiency. Since horsetail can lower vitamin B1 levels as well, taking horsetail in the setting of heavy alcohol use may worsen vitamin B1 deficiency.   
• Thiamine deficiency: Due to the enzyme thiaminase in horsetail that breaks down thiamine, taking horsetail may worsen thiamine deficiency. Your provider might want you to take a B complex or multivitamin daily if you use horsetail regularly to help prevent a thiamine deficiency.
• Pregnancy or breastfeeding: There is insufficient research on the safety of horsetail use when pregnant or breastfeeding. 
• Diabetes: Horsetail is thought to cause hypoglycemia (low blood sugar). Low blood sugar levels can be life-threatening if not treated appropriately. 
• Low potassium (hypokalemia): Horsetail has a diuretic effect. When you lose a lot of fluid from your body, you also lose a vital nutrient called potassium. Low potassium levels can be dangerous because they can cause an irregular heartbeat.
• Carrot allergy: If you have an allergy to carrots, you might also have an allergy to horsetail. 
• Nicotine allergy: Since horsetail contains a small amount of nicotine, you might also have an allergic reaction to horsetail if you have an allergy to nicotine.

Dosage: How Much Horsetail  Should I Take?

Here are a few examples of how much horsetail might be used for different conditions. However, talk to your healthcare provider about the benefits and the dosage that's right for you:

• Rheumatoid arthritis: One randomized controlled trial showed a horsetail mixture (15 milliliters three times per day) along with methotrexate (7.5 milligrams once a week) and diclofenac sodium (25 milligrams three times per day) to be superior at decreasing inflammation compared to Western medicine therapy alone.
• High blood pressure: Horsetail extract at a daily dose of 900 milligrams was found to have a similar blood pressure–lowering effect as hydrochlorothiazide at a daily dose of 25 milligrams in people with stage 1 hypertension.
• Wound healing: A double-blind clinical trial on 108 postpartum (following childbirth) nulliparous women (those who had never given birth before) showed the application of 3% E. arvense ointment to the incision site of an episiotomy (surgical cut made near the vagina during childbirth) twice a day for 10 days promoted wound healing and relieved pain.

What Happens If I Take Too Much Horsetail?

An overdose can occur if you take an amount larger than recommended. For horsetail, the maximum recommended dose is 900 milligrams per day.

If you experience irregular heartbeat (due to low potassium level) or symptoms of nicotine overdose (of note, horsetail contains a small amount of nicotine), you may have taken too much horsetail.

In terms of toxicity, it is essential to note that another horsetail species, Equisetum palustre, is poisonous to horses.

Interactions

If you are taking certain medications or other supplements, horsetail could affect how they work. You should talk with your healthcare provider before using horsetail, especially if you are taking: 

• Antidiabetic drugs: Because horsetail might lower your blood sugar levels, Actos (pioglitazone), Amaryl (glimepiride), Avandia (rosiglitazone), Diabeta (glyburide), GlucotrolXL (glipizide), insulin, and other medicines used to treat diabetes should not be taken with horsetail. Taking it with insulin or additional blood sugar–lowering drugs can result in dangerously low blood sugar levels. 
• Diuretics: Medications that flush fluid from your body can also decrease your potassium levels. Examples of these medications are Diuril (chlorothiazide), Lasix(furosemide), Microzide(hydrochlorothiazide), and Thalitone (chlorthalidone). As horsetail is a known diuretic, using horsetail with another fluid-flushing medication may cause your potassium levels to drop too low.
• Lithium: Horsetail may decrease your body's ability to eliminate lithium (a mood stabilizer), increasing the risk of lithium buildup in the body and severe side effects.  
  
• Lanoxin (digoxin): Horsetail acts like a water pill and thus theoretically can lower your potassium levels. Low potassium levels can affect the beating of your heart and increase the risk of the toxicity of the heart medication digoxin. Nevertheless, a randomized, double-blind clinical trial showed that potassium was not significantly eliminated from human urine after horsetail treatment. More studies are needed to clarify the action of horsetail concerning potassium.
• Nicotine patches or nicotine gum: Since horsetail contains a small amount of nicotine, you should not use it if you use nicotine gum or nicotine replacement patches.
• Nucleoside reverse transcriptase inhibitors (NRTIs): This medication class is used to treat human immunodeficiency virus (HIV). Taking horsetail with NRTIs may reduce the effects of these drugs. 

It is essential to carefully read a supplement's ingredient list and nutrition facts panel to know which ingredients and how much of each ingredient is included. Please review this supplement label with your healthcare provider to discuss potential interactions with foods, other supplements, and medications.

How to Store Horsetail

As with prescription medications, herbal products need to be stored properly to maintain freshness. All preparations of horsetail should be kept in a sealed dark container, away from direct sunlight. Discard as indicated on the packaging. Keep this and all supplements and medications out of the reach of children and pets.

Similar Supplements

The following supplements may have similar effects and properties as horsetail:

• Aloe
• Bitter melon
• Cinnamon
• Chromium
• Prickly pear cactus
• Bilberry
• Brewer's yeast
• Cascara sagrada

Like horsetail, aloe, bitter melon, cinnamon, chromium, prickly pear cactus, and bilberry may also lower blood sugar levels. Therefore, it is best not to combine horsetail with such supplements to prevent blood sugar levels from dropping too low. 

Furthermore, since horsetail contains chromium, taking horsetail with chromium-containing supplements such as brewer's yeast and cascara sagrada can increase the risk of chromium poisoning.

Sources of Horsetail & What to Look For

Horsetail is available as a supplement in various forms, including liquids, tinctures, capsules, topical ointment, or tea. You can make horsetail tea by using teabags or a loose herb. 

In general, opt for products that have good manufacturing practices (GMP) or third-party lab-tested certifications.

Horsetail Supplements

While some horsetail products are labeled “thiaminase-free,” it is unclear what the side effects might be.

Summary

Horsetail is a plant that has been used for thousands of years. Research has shown that horsetail may play a role in promoting bone formation, decreasing inflammation, lowering blood pressure, lowering blood sugar, and promoting hair growth.  However, more human studies are needed to clarify the efficacy and safety of horsetail.