Comfrey and Pyrrolizidine Alkaloids: Why the FDA Restricted the Herb

Comfrey (Symphytum officinale) has been used in folk medicine for over two thousand years, often as a topical for wound healing and as a tea for cough and joint pain. Its enduring presence in the herbal aisle obscures a serious problem: comfrey contains pyrrolizidine alkaloids (PAs), a class of compounds that human liver enzymes convert into reactive metabolites capable of causing hepatic veno-occlusive disease, cirrhosis, and liver cancer. The FDA banned oral comfrey products in 2001. The herb remains in some topical preparations, and confusion persists.

The chemistry that makes comfrey toxic

Comfrey root and leaf contain at least 14 different pyrrolizidine alkaloids, including echimidine, intermedine, lycopsamine, and symphytine. Hepatic CYP3A4 oxidises these compounds into pyrrolic esters that covalently bind to DNA and protein, particularly in hepatic sinusoidal endothelial cells. The injury manifests as hepatic sinusoidal obstruction syndrome (SOS) — formerly called hepatic veno-occlusive disease — which causes painful hepatomegaly, ascites, and progressive liver failure [1].

The case series that drove regulatory action

Multiple case reports from the 1980s and 1990s documented SOS in patients drinking comfrey tea, eating comfrey supplements, or applying very high topical doses. A 2001 review in the Lancet compiled 16 published cases — most progressing to cirrhosis and several requiring liver transplantation [2]. The FDA's July 2001 advisory recommended removal of comfrey-containing oral products from the US market, citing both acute hepatotoxicity and PA carcinogenicity in long-term rodent studies [3].

The international regulatory pattern

Germany's BfArM banned internal use of comfrey in 1992 and limits topical exposure to no more than 100 µg of total PA per day for no longer than 4–6 weeks. The UK MHRA restricts oral comfrey products. Canada and Australia have similar restrictions. The European Medicines Agency's herbal monograph on comfrey root permits topical use only on intact skin for short durations [4]. The consensus across regulators is that oral comfrey is unsafe at any dose.

The "PA-free comfrey" claim

Some products are marketed as "PA-free comfrey" using cultivars with reduced alkaloid content or supercritical CO₂ extraction. Independent analysis of marketed products has shown variable PA content, with some products labelled as PA-free still containing detectable alkaloid levels [5]. The German PA limit (1 µg total PA per day for foods) is below the level reliably achievable in some commercial extracts. Anyone using topical comfrey should restrict use to no more than 4–6 weeks per year, apply only to intact skin, and avoid use during pregnancy and breastfeeding.

What about contamination of unrelated herbs

PAs are not unique to comfrey. They occur in Senecio, Crotalaria, Heliotropium, and Borago species (borage), and as contaminants in honey, herbal teas, and grain crops that grow next to PA-producing weeds. Several outbreaks of SOS in developing countries have traced to contaminated wheat. European regulators have established PA limits in foods and herbal teas (1 µg/day chronic exposure threshold) precisely because of this widespread contamination [6].

Topical comfrey: where the modest benefit lies

Topical comfrey extracts have demonstrated efficacy for acute musculoskeletal pain — sprains, strains, contusions — in randomised trials. A 2012 systematic review of 7 RCTs found topical comfrey reduced pain compared to placebo, with effect sizes comparable to topical NSAIDs [7]. The benefit is real but does not extend to oral preparations, and short-term topical use is what European regulators tolerate.

Practical takeaway

Oral comfrey should not be used under any circumstance. Comfrey tea, capsules, and tinctures should be discarded if found in the cupboard. Topical comfrey for acute sprains is reasonable for 1–2 weeks of use; do not apply to broken skin, do not use during pregnancy, and do not exceed the labelled duration. If a herbal product mentions "Symphytum," check whether it is for internal use and avoid if so. Anyone with unexplained liver enzyme abnormalities should be asked specifically about comfrey and other PA-containing botanicals.