Horsetail has a plausible biological rationale for supporting hair health, mainly through its silicon content, but there is no published clinical trial showing that horsetail alone makes human hair grow. The evidence that exists is a patchwork of lab studies on horsetail extracts, clinical trials of multi-ingredient supplements that happen to include it, and a modest body of research on bioavailable silicon. That patchwork is suggestive enough to keep the ingredient popular in hair supplements, but thin enough that no regulatory body has approved a hair-growth claim for it.
Why People Think Horsetail Should Work
Horsetail (Equisetum arvense) is one of the most silicon-rich plants you can buy. Silicon is a trace element found in connective tissues throughout the body, including skin, nails, and hair. The logic goes like this: hair is built partly from structural proteins that benefit from adequate silicon, horsetail is loaded with silicon, so taking horsetail should strengthen or thicken hair. That logic is not unreasonable, but each link in the chain is weaker than supplement marketing suggests.
Beyond silicon, horsetail contains flavonoids and other phenolic compounds in meaningful quantities.1International Journal of Biology and Chemistry. Chemical composition and potential pharmacological properties of field horsetail extract based on GC-MS analysis These compounds have antioxidant properties, and some researchers have explored whether they contribute to scalp health. But the headline ingredient in almost every hair-focused horsetail product is silicon.
What Silicon Actually Does to Hair
The strongest piece of evidence linking silicon to hair comes from a trial that did not use horsetail at all. It used choline-stabilized orthosilicic acid, a highly bioavailable form of silicon, given orally to women with fine hair over nine months. Compared to placebo, the silicon group had thicker hair strands and better tensile strength, meaning the hair resisted stretching and breaking more effectively. The cross-sectional area of individual hairs increased in the supplement group but not in the placebo group, and the change in urinary silicon excretion correlated with hair thickness gains.2PubMed. Effect of oral intake of choline-stabilized orthosilicic acid on hair tensile strength and morphology in women with fine hair
This is a real, placebo-controlled finding, and it tells us that getting more bioavailable silicon into the body can improve hair quality. The catch is that choline-stabilized orthosilicic acid is a pharmaceutical-grade silicon supplement engineered for absorption. It is not the same as grinding up horsetail and swallowing it. Whether the silicon locked inside a plant cell wall behaves the same way in your gut is a separate question entirely, and the answer is not encouraging.
The Bioavailability Problem
Silicon in horsetail exists largely as silica, bound up in the plant’s rigid cell walls. Your body cannot absorb silica the way it absorbs dissolved orthosilicic acid. A pilot crossover study comparing several dietary silicon supplements found that even purpose-built formulations only delivered roughly 27 to 35 percent of their silicon dose into the bloodstream, as measured by urinary excretion over six hours.3Scientific Reports. Relative absorption of silicon from different formulations of dietary supplements: a pilot randomized, double-blind, crossover post-prandial study Those were supplements specifically designed to be absorbable. Raw horsetail extract, where the silicon is trapped in an amorphous or crystalline silica matrix, is likely absorbed far less efficiently.
Processing conditions dramatically affect how much silica ends up in a horsetail product, too. Analysis of different horsetail species and extraction methods has shown that the silicon dioxide content of ash residues can range anywhere from about 32 to 98 percent depending on how the raw material was handled. Different species of horsetail also vary: field horsetail (the one sold in supplements) actually contains less silica than forest or wintering horsetail species.4Izvestiya Vuzov. Prikladnaya Khimiya i Biotekhnologiya. Silicon-containing compounds in horsetail (Equisetum Equisetaceae) composition So two horsetail supplements on the same shelf can deliver very different amounts of silicon, and neither label will tell you how much your body will actually absorb.
Lab Evidence on Hormonal Pathways
One of the more interesting lines of research involves a related species, Equisetum debile. In lab testing, extracts of this horsetail inhibited 5α-reductase, the enzyme that converts testosterone into dihydrotestosterone (DHT). DHT is the hormone most directly linked to androgenetic alopecia, the common pattern hair loss that affects both men and women. The same extracts also reduced secretion of the inflammatory signaling molecule IL-6 and showed antioxidant activity against lipid peroxidation.5PubMed Central. Inhibition of 5α-Reductase, IL-6 Secretion, and Oxidation Process of Equisetum debile Roxb. ex Vaucher Extract as Functional Food and Nutraceuticals Ingredients
Separately, laboratory work on herbal extracts applied to cultured human dermal papilla cells (the cells at the base of hair follicles that regulate growth) found that certain herbal mixtures could stimulate those cells to proliferate and could reduce expression of 5α-reductase. The extracts also affected signaling proteins involved in cell growth and survival.6PubMed Central. Herbal Extracts Induce Dermal Papilla Cell Proliferation of Human Hair Follicles These findings are genuinely interesting from a mechanistic standpoint. But cells in a dish respond to concentrations of compounds that may never reach the follicle when you take a capsule. The gap between “inhibits an enzyme in a lab assay” and “slows hair loss on your scalp” is enormous, and no human trial has bridged it for horsetail specifically.
What the Supplement Trials Actually Tested
If you search for clinical evidence that horsetail helps hair growth, you will find several placebo-controlled trials that seem to support the claim. Look closer, though, and a pattern emerges: none of these trials tested horsetail alone. They tested multi-ingredient oral supplements that included marine proteins, biotin, zinc, vitamin C, iron, horsetail extract, and other compounds in various combinations.
One double-blind trial in women with self-perceived thinning hair found that after 180 days, the supplement group’s terminal hair count in a target scalp area rose dramatically compared to placebo, and subjects reported improvements in hair volume, scalp coverage, and thickness.7PubMed Central. A Double-blind, Placebo-controlled Study Evaluating the Efficacy of an Oral Supplement in Women with Self-perceived Thinning Hair A follow-up extension trial confirmed significant increases in terminal hair count and hair diameter at both 90 and 180 days compared to placebo.8PubMed Central. A Randomized, Double-blind, Placebo-controlled, Multi-center, Extension Trial Evaluating the Efficacy of a New Oral Supplement in Women with Self-perceived Thinning Hair A separate study of a marine protein supplement in women with temporary hair thinning also found increased terminal hair counts and reduced shedding over 90 days.9PubMed Central. A 3-Month, Randomized, Double-Blind, Placebo-Controlled Study Evaluating the Ability of an Extra-Strength Marine Protein Supplement to Promote Hair Growth and Decrease Shedding in Women with Self-Perceived Thinning Hair
These results are real, and they are encouraging for the multi-ingredient formulations tested. The problem is that you cannot attribute the results to horsetail specifically. When a supplement contains a marine protein complex, biotin, zinc, vitamin C, and horsetail extract, and hair grows, you have no way of knowing which ingredient did the work, or whether the combination matters more than any single component. It is entirely possible that the biotin and marine protein carried the effect while horsetail was along for the ride. It is also possible that horsetail contributed. The trial designs simply cannot tell us.
A review of popular oral hair supplement ingredients noted this exact issue: while some ingredients in these formulations have strong standalone evidence, many have never been tested in isolation in clinical trials or have only in vitro or animal data supporting hair growth.10PubMed Central. Clinical efficacy of popular oral hair growth supplement ingredients Horsetail falls squarely in the category of ingredients that ride in the mix but have not been isolated and proven on their own.
Safety Is Not a Given
Because horsetail is a plant, people tend to assume it is harmless. It is mostly well tolerated at typical supplement doses, but there are real risks worth knowing about, especially with long-term use.
The most established concern involves thiaminase, an enzyme naturally present in horsetail that breaks down thiamine (vitamin B1). Chronic use of horsetail supplements can deplete B1, particularly if your diet is already marginal or if you consume alcohol regularly. A case report documented a woman who took 1,200 mg per day of horsetail for over a year before and during pregnancy, contributing to a significant B-complex vitamin deficiency that the authors connected to the thiaminase activity of the supplement combined with alcohol exposure.11PubMed Central. Prenatal exposure of a girl with autism spectrum disorder to ‘horsetail’ (Equisetum arvense) herbal remedy and alcohol: a case report This is an extreme case, but it illustrates a mechanism that can affect anyone taking horsetail at high doses over months.
Drug interactions are another concern. Horsetail has known diuretic properties, and case reports have documented possible interactions with antiretroviral medications. In two patients on long-standing HIV treatment regimens, starting a horsetail supplement was associated with virological breakthrough, meaning the previously effective antiviral drugs appeared to lose efficacy.12PubMed. Possible Drug-Herb Interaction between Herbal Supplement Containing Horsetail (Equisetum arvense) and Antiretroviral Drugs The authors noted that pharmacological data on horsetail’s interactions with other drugs remain limited. If you take prescription medications, mentioning horsetail use to your prescriber is worth doing.
The Species Mix-Up Problem
There is a quality control issue in the horsetail supplement market that most consumers are unaware of. The species sold for supplementation is supposed to be Equisetum arvense, field horsetail. But another species, Equisetum palustre (marsh horsetail), looks almost identical and grows in similar habitats. Marsh horsetail contains palustrine, a potentially toxic alkaloid that field horsetail does not. The European Pharmacopoeia requires that horsetail products consist of at least 95 percent E. arvense, with no more than 5 percent foreign material including E. palustre, but analytical testing has confirmed that the two species are frequently confused during harvesting.13PubMed. Biomarker-Based Determination of Equiseti herba Contamination by Equisetum palustre Using HPLC-MS/MS
Researchers have developed HPLC-based methods to detect contamination, but these are laboratory techniques, not something printed on a consumer label. Cheap or poorly sourced horsetail products may contain meaningful amounts of the wrong species without anyone knowing. This is not a reason to panic, but it is a reason to buy from brands that do third-party testing or follow pharmacopoeia standards, and to be skeptical of bargain-bin herbal capsules.
Who Might Still Benefit
If you are genuinely silicon-deficient, there is a reasonable case that increasing your silicon intake could improve hair quality. The clinical trial on choline-stabilized orthosilicic acid showed real improvements in hair thickness and tensile strength.2PubMed. Effect of oral intake of choline-stabilized orthosilicic acid on hair tensile strength and morphology in women with fine hair But the key word there is “bioavailable.” If your goal is to get more silicon to your hair follicles, a supplement designed for absorption will almost certainly outperform a dried herb whose silicon is locked in plant silica.
If your hair thinning is driven by androgenetic alopecia (pattern baldness), the lab evidence on 5α-reductase inhibition from Equisetum species is interesting but far too preliminary to compete with proven treatments like finasteride or minoxidil. Those drugs have decades of clinical trial data behind them. Horsetail has cell culture assays. The comparison is not close.
If your thinning is related to nutritional deficiency, stress, or a temporary condition like telogen effluvium (where a shock to the system pushes many hairs into the shedding phase at once), a well-formulated multi-ingredient supplement that includes horsetail alongside biotin, marine proteins, and other nutrients might help. The clinical trials of these combination products did show real results in women with self-perceived thinning hair.8PubMed Central. A Randomized, Double-blind, Placebo-controlled, Multi-center, Extension Trial Evaluating the Efficacy of a New Oral Supplement in Women with Self-perceived Thinning Hair Just remember that horsetail is one piece of those formulations, and possibly not the most important one.
Topical Horsetail Products
Alongside capsules and teas, you will find horsetail-infused shampoos, serums, and scalp treatments. These products face an even steeper evidence gap than oral supplements. No controlled trial has tested a topical horsetail-only product for hair growth in humans. The theoretical appeal is that applying the extract directly to the scalp might deliver its flavonoids and antioxidants where they are needed, but whether those compounds penetrate the skin at meaningful concentrations is unknown.
Topical products do have one advantage over oral supplements: they sidestep the bioavailability problem for silicon, because the goal is local delivery rather than systemic absorption. But they introduce their own unknowns about skin penetration and stability of the active compounds in a cosmetic base. For now, if a horsetail shampoo makes your hair feel nice, that is probably the cosmetic formulation at work rather than the horsetail accomplishing anything pharmacological.
How Horsetail Supplements Vary on the Shelf
Walk down the supplement aisle and you will see horsetail products ranging from dried herb capsules to liquid extracts to standardized silica tablets. The variation matters more than most consumers realize. As the analytical work on different species and processing methods showed, silicon content swings wildly depending on which species of horsetail was used and how it was processed.4Izvestiya Vuzov. Prikladnaya Khimiya i Biotekhnologiya. Silicon-containing compounds in horsetail (Equisetum Equisetaceae) composition A product standardized to a certain percentage of silica has at least gone through the step of measuring what it contains. A product that just lists “horsetail herb 500 mg” tells you almost nothing about how much silicon is actually in it or how much your body can use.
Some manufacturers extract the silicon from horsetail and convert it to a more bioavailable form before putting it in a supplement. Others simply grind the dried plant and capsule it. These are fundamentally different products wearing the same label. If you are taking horsetail specifically for its silicon content, checking whether the product is standardized, and to what compound, is the bare minimum due diligence. Even then, the absorption data suggest you would get more usable silicon from a dedicated orthosilicic acid supplement than from any horsetail product currently available.
What Regulators Have Said
No major regulatory body has approved a health claim linking horsetail to hair growth. The European Food Safety Authority has evaluated silicon-related health claims and generally found the evidence insufficient to establish a cause-and-effect relationship for the claimed benefits. In the United States, the FDA does not evaluate dietary supplements for efficacy before they reach the market, so the “supports healthy hair” language you see on bottles is a structure-function claim that requires no clinical proof. The gap between what the label implies and what the science supports is wide, and it is worth keeping that in mind when evaluating any supplement marketed for hair.