Comfrey
Symphytum officinale
A traditional 'knitbone' herb with good clinical evidence as a topical ointment for sprains, back pain, and osteoarthritis — but it contains liver-toxic pyrrolizidine alkaloids and must never be taken internally.
At a Glance
- Best known for
- Ankle sprain pain and swelling (topical, RCT evidence), Acute back pain (topical, RCT evidence), Knee osteoarthritis pain (topical)
- Evidence
- Moderate
- Typical dose
- 35% root ointment, 2 g applied 4x daily
- Type
- Herb
What is Comfrey?
Comfrey is a hardy perennial with large hairy leaves and purple bell-shaped flowers, native to Europe and western Asia. Its old English name 'knitbone' and the genus name Symphytum (from the Greek for 'to unite') both reflect its centuries-old use as a poultice for broken bones, sprains, and wounds. Modern research has confirmed real topical benefits, but it has also shown that comfrey root and leaf contain pyrrolizidine alkaloids that damage the liver when absorbed in quantity. As a result, comfrey is now used almost exclusively as an external preparation, and the best-studied commercial products are made from root extracts with the alkaloids removed or reduced to trace levels.
Known Health Benefits
How It Works
Allantoin stimulates cell proliferation and is thought to speed the regrowth of skin and connective tissue, which is why it also appears in many cosmetic wound-care products. Rosmarinic acid and related phenolic compounds inhibit inflammatory enzymes and reduce prostaglandin production, providing a local analgesic and anti-inflammatory effect. The mucilage forms a soothing, moisture-retaining film on the skin. Tannins have a mild astringent effect that may reduce swelling. The pyrrolizidine alkaloids play no beneficial role; they are converted by liver enzymes into reactive compounds that damage the small veins of the liver, which is the basis of comfrey's toxicity when swallowed.
What Research Says
Comfrey is one of the better-studied topical herbs. A randomized, double-blind trial of 142 people with acute ankle sprain found a 35% comfrey root ointment reduced pain and swelling as effectively as diclofenac gel. A separate RCT of 120 people with acute upper or lower back pain reported that comfrey root extract ointment reduced pain intensity by about 95% over five days, compared with roughly 38% with placebo — an unusually large effect that would benefit from independent replication. In knee osteoarthritis, a 3-week RCT of 220 patients found significant pain reduction and improved mobility versus placebo. A small trial in children and adults with fresh abrasions found faster wound healing with a PA-free comfrey cream. Most of these trials were conducted by or in collaboration with manufacturers of a single standardized German product, and durations were short (5–21 days). The evidence supports short-term topical use; it does not support oral use, which carries documented risk of serious liver injury.
Active Compounds
Allantoin, rosmarinic acid, mucilage polysaccharides, tannins, choline; also toxic pyrrolizidine alkaloids (echimidine, symphytine)
Forms & Bioavailability
Topical application delivers allantoin and rosmarinic acid into the skin and underlying tissue. Pyrrolizidine alkaloids can also cross intact skin to a small degree, which is why regulators limit daily topical use and duration even for external products, and why PA-free extracts are preferred. Absorption is greater through broken skin. Orally, the alkaloids are readily absorbed and activated in the liver, and the resulting damage is cumulative, so even small repeated doses are a concern.
Dosage Guidance
| Use Case | Dosage |
|---|---|
| Ankle sprain | 35% root ointment, 2 g applied 4x daily |
| Acute back pain | 4 g ointment 3x daily |
| Knee osteoarthritis | 6 g ointment 3x daily |
| Minor abrasions | PA-free cream, thin layer 2–3x daily |
| Oral use | Not recommended |
Always consult a healthcare provider for personalized dosing.
Potential Side Effects
Topical: occasional skin redness or irritation. Oral: hepatic veno-occlusive disease, liver failure, possible carcinogenicity — oral use is banned or restricted in many countries
Who Should Avoid It
- Any oral or internal use (risk of hepatic veno-occlusive disease and liver failure)
- Application to open wounds or broken skin unless the product is certified PA-free
- Pre-existing liver disease (avoid even topical use or keep to minimal areas)
- Children under 3 (and limit topical use in older children to short courses)
- Topical use beyond 4–6 weeks per year (regulatory guidance to limit cumulative alkaloid exposure)
Pregnancy & Lactation
Comfrey should be avoided in pregnancy and breastfeeding in all forms. Pyrrolizidine alkaloids cross the placenta and have caused liver damage in the fetus in animal studies and case reports, and they can pass into breast milk. Even topical use is discouraged because small amounts are absorbed through the skin. Choose alternatives such as arnica gel (on intact skin) or calendula cream for minor bruises and skin irritation during this period.
Known Drug Interactions
Oral comfrey adds to the liver toxicity of hepatotoxic drugs (acetaminophen, methotrexate, statins, alcohol); topical PA-free products have no established drug interactions
Evidence Classification
Supported by cohort studies, case-control studies, or multiple observational studies with consistent findings.
Frequently Asked Questions
Why is comfrey banned for internal use if it has been used for centuries?
Traditional use did not reveal the slow, cumulative liver damage caused by pyrrolizidine alkaloids, which can take months or years to show up as hepatic veno-occlusive disease. Once case reports and animal studies linked comfrey tea and capsules to liver failure and tumors, regulators in the US (2001), UK, Canada, and EU prohibited oral comfrey products. The topical benefits are real; the oral risks are simply unacceptable.
Is a comfrey poultice from my garden safe to use?
A fresh-leaf poultice on intact skin for a few days carries low risk, but homemade preparations contain the full complement of alkaloids, some of which penetrate the skin. Avoid using it on broken skin, over large areas, for more than a week or two, or on children. Commercial PA-free ointments provide the benefits with much less alkaloid exposure.
How does comfrey compare to arnica?
Both are topical anti-inflammatories for sprains and muscle pain. Comfrey has the stronger trial evidence, including a head-to-head study against diclofenac gel for ankle sprains, and it has the advantage of PA-free versions being usable on minor abrasions. Arnica has a longer safety track record on intact skin and no concerns about liver toxicity. Many people use arnica for bruises and comfrey for sprains and joint pain, but either is reasonable for short-term use.
What does 'PA-free' mean on a comfrey product?
PA stands for pyrrolizidine alkaloids, the toxic compounds in comfrey. PA-free (or PA-reduced) products are made from specially processed root extracts or alkaloid-poor cultivars, with alkaloid levels below detection limits. These are the preparations used in most of the clinical trials and are the only comfrey products that should be applied to broken skin.
References
- Efficacy and tolerance of a comfrey root extract (Extr. Rad. Symphyti) in the treatment of ankle distorsions: results of a multicenter, randomized, placebo-controlled, double-blind study. Koll R, Buhr M, Dieter R, et al.. Phytomedicine (2004)
- Comfrey root extract ointment in the treatment of acute upper or lower back pain: results of a double-blind, randomised, placebo-controlled, multicentre trial. Giannetti BM, Staiger C, Bulitta M, Predel HG. British Journal of Sports Medicine (2010)
- Comfrey: a clinical overview. Staiger C. Phytotherapy Research (2012)
- FDA advises dietary supplement manufacturers to remove comfrey products from the market. U.S. Food and Drug Administration (2001)
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This entry is for educational purposes only. It is not medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement regimen, especially if you take medications or have health conditions.