Why "Comfrey is Toxic" is Only Half the Story

The Rat Skin Study · Doing the Math · Root vs. Leaf · Species Matters Too · Extraction Method Matters · Where Real Caution Still Belongs · The Bottom Line

If you've spent any time reading about comfrey, you've run into the warning: pyrrolizidine alkaloids (PAs), liver damage, "don't use this." Most of that warning is real — but it's almost entirely about eating comfrey, not rubbing it on your skin. The two uses get very intertwined, and the details that actually matter get lost. So we have broken down the real evidence, studies, and add a little math below.

Here's what the research says when you separate them out.

The Rat Skin Study People Keep Citing

The study almost everyone points to is Brauchli et al., 1982 — the one usually summarized as "comfrey harmed rats when applied to their skin." That's not quite what it did.

Researchers shaved a patch of skin on rats and applied a crude alcoholic extract of comfrey root (not leaf) for 44 hours. They were checking one thing: how much of the alkaloid content would show up in the rats' urine, meaning it had crossed the skin and entered circulation.

The answer: 0.1–0.4% of the applied dose. A small fraction made it through, and of that fraction, very little converted into the more dangerous "free alkaloid" form — most stayed in the safer N-oxide form the body clears more easily.

What the study did not do is look at liver injury, tumors, or any other toxic outcome. It measured absorption, full stop. For comparison, when the same alkaloids were given orally in other research, urinary levels came out 20–50 times higher — which tracks, since the gut is built to absorb things and skin is built to keep them out.

The World Health Organization's own toxicology review of this research describes it the same way: an absorption study, not a harm study.

So the accurate summary is: a small amount of comfrey alkaloid can cross skin. The dermal study never showed that this crossing caused harm. Every well-documented case of comfrey-related liver toxicity comes from ingestion — tea, capsules, or other oral use — not from topical application.

Doing the Math: What the "Toxic Dose" Actually Looks Like in Real Life

Numbers on paper don't mean much until you scale them to a person. So let's actually run them.

The chronic liver-tumor study. The most-cited carcinogenicity data (Hirono et al., 1978) fed rats a diet that was 2% comfrey root for up to 600 days — close to that rat's entire adult life — and found a 42% liver tumor rate.

Run the numbers:

A rat eats roughly 5% of its body weight in food per day. A 200g rat eating 10g of food a day, at 2% comfrey, works out to about 200mg of dried comfrey root every single day, continuously, for the better part of two years.
That's 1,000mg per kg of body weight, per day — every day, for most of the animal's life.
Using the standard interspecies dose conversion regulators use to translate a rat dose into a human-equivalent dose, that scales to roughly 160mg per kg for a human — or about 11 grams of dried comfrey root, every day, for years, for a 70kg adult.

Eleven grams a day is a genuinely enormous amount. A typical dried-herb capsule holds 300–500mg, so that's on the order of 25–35 capsules a day, every day, indefinitely — not a supplement routine anyone follows, and nowhere close to how a salve is used.

The tea comparison. A separate analysis (Roitman, 1981) found that a single cup of commercially available comfrey tea contained about 26mg of PAs. That's a striking number on its own, and it's exactly why oral comfrey — tea, capsules, tinctures you drink — is the form that's actually restricted in most countries. It has nothing to do with skin application.

The reality-check version. One independent review of the comfrey literature ran the numbers a different way: to reach a meaningful dose of just one comfrey alkaloid (symphytine), a person would need to eat somewhere between 7 and over 1,000 kilograms of dry leaf, depending on the specific plant sample. Nobody is doing that by rubbing on a salve.

None of this erases the alkaloids. It just puts the actual toxic doses in context — they were reached through sustained, high-volume oral exposure, which we want to mention as this go throughs the liver directly.

Root vs. Leaf: What the Numbers Actually Show

Comfrey root and comfrey leaf are not interchangeable when it comes to PA content — they're genuinely different raw materials.

Pyrrolizidine alkaloids. The comparison most often cited in the research (Couet et al., 1996, referenced directly in the European Medicines Agency's own assessment report on comfrey) tested commercial comfrey samples and found:

Root: 1,380–8,320 µg/g (roughly 0.14%–0.83% by weight)
Leaf: 15–55 µg/g (roughly 0.0015%–0.0055% by weight)

That's leaf coming in at somewhere around 1–4% of the root's alkaloid load — often far less. Other labs using different extraction methods have found wider ranges (root as low as 100 µg/g in some samples, leaf as high as 2,200 µg/g in isolated outliers), but the pattern holds across the literature: root is consistently higher, and root is also far more erratic — swinging widely by harvest time, soil, and plant age. A 2023 Polish HPLC-MS/MS study comparing root and leaf samples confirmed this directly: leaf carries a lower PA load and a noticeably more stable, predictable profile from batch to batch, while root varies widely enough that some commercial root samples tested well above what's considered a safe daily exposure.

It's also worth noting that the European regulatory monograph restricting comfrey to short-term, intact-skin-only use is specifically about comfrey root — that's the plant part regulators were looking at when those limits were written, and it's the part with the highest, least consistent alkaloid content to begin with.

Species Matters Too — Comfrey Isn't Just One Plant

Here's something most comfrey warnings skip entirely: "comfrey" isn't one plant. It's a genus — Symphytum — and the toxicology literature usually breaks it into five species that cover basically all commercial and traditional use. A 2010 review of comfrey's metabolism and carcinogenicity uses exactly this framework, and for good reason — treating "comfrey" as one uniform thing hides some pretty large differences:

Common comfrey (Symphytum officinale) — the species behind most of the research already covered here. Root PA content is reported all over the map depending on who's measuring: one widely cited commercial-sample study (Couet et al., 1996) puts it at 1,380–8,320 µg/g, while another lands on a much narrower 0.25–0.29%. The leaf sits far lower and far more consistently, around 15–55 µg/g. Even the root-to-leaf ratio isn't agreed on — the older rule of thumb (Mattocks, 1986; Roitman, 1981) says root runs about ten times leaf, while the Couet numbers imply something closer to a hundredfold gap.
Prickly comfrey (S. asperum) and Russian comfrey (S. × uplandicum) — a hybrid of common and prickly comfrey — are where things actually get worse. Both carry echimidine, generally considered the nastiest of the comfrey alkaloids, as a normal part of their chemistry — something true common comfrey mostly doesn't have. A 2013 review of comfrey's external uses says it outright: S. officinale runs lower in PAs than S. × uplandicum. Russian comfrey isn't some rare specialty plant, either — in places like the UK, it's actually the more commonly cultivated "comfrey," bred for being hardier and faster-growing than the true species. So a fair amount of commercial "comfrey" out there was never the lower-alkaloid plant to begin with.
Caucasian comfrey (S. caucasicum) — valued traditionally for wound care because of its allantoin content, but also one of the species researchers have found the most pyrrolizidine alkaloids in, right alongside prickly comfrey.
Tuberous comfrey (S. tuberosum) — the outlier at the low end. It grows across South-East and Central Europe and is genuinely eaten there: the root gets roasted as a coffee substitute, the leaves and shoots get foraged, and people are still doing this today. Independent sources keep landing on roughly the same number — total PA content around 0.02%, which sits below common comfrey root by a wide margin and roughly matches or slightly exceeds common comfrey leaf, but it actually has all of its plant from the root and leaf all in this total number.

None of this is theoretical, either. In 1988, Canadian regulators tested 13 commercial comfrey products, every single one labeled just "comfrey" or specifically "Symphytum officinale." Six of the thirteen tested positive for echimidine — meaning six products labeled as the lower-risk species were actually made from prickly or Russian comfrey. Three of those six even had "Symphytum officinale" printed on the label. One root product turned out to be over 85% echimidine by HPLC. There's an older story behind this too: a 1968 Japanese study on comfrey alkaloids identified its plant material as S. officinale when it was almost certainly a Russian comfrey hybrid, and that mistake got cited by later researchers as if it were real data about true common comfrey — muddying the literature for years.

Overall this is not something a "comfrey" label actually tells you. Leaf from true common comfrey, root from common comfrey, tuberous comfrey, and Russian or prickly comfrey are four genuinely different raw materials — and any of them could show up under the same generic word on a package. Knowing (and ideally verifying) which one is actually in the jar matters at least as much as the root-vs-leaf question already covered.

Extraction Method Matters More Than People Realize

Here's the part that gets skipped in almost every online comfrey warning: how the plant is processed changes what actually ends up in the final product.

Comfrey's PAs exist mostly as N-oxides — polar, water- and alcohol-soluble compounds. They are not fat-soluble. This is basic solvent chemistry, and it's why every lab study that quantifies PA content — including Brauchli's — uses alcohol or alcohol-water mixtures to pull the alkaloids out of the plant. Analytical chemists measuring PA levels routinely use methanol-water solvent specifically because it's efficient at extracting these compounds. Oil is a poor solvent for them.

That distinction matters a lot for how a product is actually made:

Small-batch, oil-infused comfrey products — the kind where dried leaf sits in a carrier oil (olive, sunflower, meadowfoam, etc.) at room temperature or gentle warmth for several weeks — are drawing out the fat-soluble constituents: waxes, chlorophyll, and other lipid-compatible compounds. That slow infusion process is a poor match for pulling water-soluble alkaloid salts out of the plant material.
Alcohol tinctures, alcohol-based extracts, and industrial/machine extraction (the kind used in lab settings and some large-scale manufacturing) are a much better solvent match for the alkaloids. That's exactly the extraction method used in the Brauchli study — a crude alcoholic extract, not an oil infusion.

To be fully upfront: there isn't a published, peer-reviewed study that directly measured finished-product PA levels in an oil-infused comfrey salve side-by-side with an alcohol tincture. But the underlying chemistry explaining the gap — alkaloid solubility profile, and the solvent choice every PA-quantification study makes for that exact reason — is well established and consistently cited by herbal formulators.

In short: not all comfrey products are made the same way, and the method matters as much as the plant part.

Where Real Caution Still Belongs

None of this means "unlimited topical comfrey, forever, no thought required." A few situations genuinely call for more care:

Broken or significantly compromised skin. The skin barrier is what limits absorption in the first place — an open wound removes that barrier.
Pregnancy and liver disease. Anywhere the liver's margin for handling anything extra is already reduced, it's not the place to test limits.
High-PA root extracts, especially alcohol-based ones, used over large areas for extended periods. This is a different exposure profile than a leaf-based, oil-infused salve used on a joint or a scrape.
Unregulated products with no idea how they were made. If you don't know the extraction method or plant part, you don't actually know what you're applying. You should be able to reach out to the manufacturer, and they should know exactly how the extract or the product came to be.

The Bottom Line

The rat skin study that gets cited as proof comfrey is dangerous on skin was actually measuring how much crossed the skin — not whether it caused harm. It didn't. The doses tied to real liver damage were reached through sustained oral intake, at levels that, translated to a human, look like dozens of capsules a day for years — not a jar of salve.

Add in that PAs are pulled out by alcohol and water, not oil, and a slow-infused, leaf-based comfrey oil is working with a very different chemical profile than the concentrated alcoholic extracts used in the studies that raised the alarm in the first place.