Comfrey's liver risk: safe topical use for US gardeners

Quick answer: Comfrey (Symphytum officinale) contains pyrrolizidine alkaloids that pose a significant liver risk if ingested, making topical application the only safe option for US gardeners. The FDA has advised against internal use since 2001 due to potential liver damage.

In many home gardens across the US, from the humid summers of USDA zone 7 in North Carolina to the cooler climates of zone 4 in Minnesota, comfrey (Symphytum officinale) is a hardy, useful plant. This perennial herb, known for its deep taproots and rapid growth, benefits soil health and compost systems. However, its historical reputation as a medicinal herb carries an important caveat: while externally beneficial, comfrey poses serious risks if consumed internally.

For centuries, various cultures have utilized comfrey for its perceived healing properties, often referring to it as “knitbone” due to its traditional use in mending fractures and soothing bruises. Yet, modern scientific understanding, particularly since the late 20th century, has highlighted the presence of pyrrolizidine alkaloids (PAs) within the plant. These compounds, even in small amounts, can lead to severe liver damage, a fact that has prompted health advisories from regulatory bodies like the FDA since 2001. Internal use should be strictly avoided by all US gardeners.

Comfrey’s garden utility and historical medicinal uses

Comfrey (Symphytum officinale) is a workhorse in many US gardens, particularly for those looking to improve soil fertility naturally. Its deep taproots can extend up to 10 feet into the soil, drawing up nutrients like potassium, phosphorus, and calcium from depths inaccessible to most other plants. This characteristic earns it the title of a dynamic accumulator, making it an excellent candidate for chop-and-drop mulching or as a valuable addition to the compost pile. In USDA zones 3 to 9, a mature comfrey plant can produce several harvests of lush foliage each season, with leaves often reaching 6 to 12 inches in length and the plant itself growing 2 to 3 feet tall and wide.

When added to compost, comfrey leaves decompose quickly, accelerating the breakdown of other organic materials. They are a nitrogen-rich green, with a carbon to nitrogen ratio in the region of 10:1 to 15:1, which is why a layer of them heats up a carbon-heavy pile. Beyond compost, comfrey can be steeped to create a potent liquid fertilizer, often diluted to a 1:10 ratio for direct application to plants. Historically, comfrey was also known for its external medicinal applications, with references dating back to 1936 describing its use in poultices for sprains and broken bones. This dual history shows its garden value alongside the need for clear safety guidelines that emerged more fully in the late 20th century.

comfrey’s many garden roles

• Compost activator: Its high nitrogen content and rapid decomposition speed up composting.
• Nutrient accumulator: Deep roots bring up essential minerals from 10 feet down.
• Liquid fertilizer: Fermented leaves create a nutrient-rich plant feed, often diluted 1:10.
• Mulch material: Chop-and-drop leaves suppress weeds and return nutrients to the topsoil.
• Forage crop (historically only): Prickly comfrey was fed to stock in 1910. Do not feed comfrey to livestock or pets: the same pyrrolizidine alkaloids cause cumulative liver damage in horses, cattle, sheep, pigs and poultry.

Understanding pyrrolizidine alkaloids and liver toxicity

Despite its garden utility, comfrey safety centers on its pyrrolizidine alkaloids (PAs). These natural compounds, present in all parts of the Symphytum officinale plant, are potent liver toxins. When ingested, PAs can cause veno-occlusive disease of the liver, a condition where small veins in the liver become blocked, leading to severe and potentially fatal damage. Even small, repeated doses accumulate. Regulators work in micrograms, not milligrams: German authorities have set a daily oral limit of roughly 1 microgram of pyrrolizidine alkaloids, and around 100 micrograms for external preparations used no longer than four to six weeks in a year. The liver’s inability to fully detoxify these compounds makes even seemingly minor exposure a long-term concern.

The danger of internal comfrey consumption is not new. As early as 1936, concerns about comfrey’s effects were being noted, and by the late 20th century, scientific evidence solidified these warnings. In 2001, the US Food and Drug Administration (FDA) issued an advisory, recommending against the internal use of comfrey products, including teas, capsules, and tinctures, due to the undeniable risk of liver damage. This advisory, based on extensive toxicological data from multiple studies, indicates that there is no safe dosage for internal consumption. Gardeners in regions like the Pacific Northwest, where comfrey grows particularly vigorously, should be especially vigilant about preventing accidental ingestion by humans or pets.

plants containing pyrrolizidine alkaloids

• Comfrey (Symphytum officinale): All parts, especially roots.
• Borage (Borago officinalis): Leaves and flowers contain pyrrolizidine alkaloids; treat it as an occasional garnish rather than a regular green, and choose borage seed oil certified free of alkaloids.
• Tussilago (Tussilago farfara, coltsfoot): Used historically but contains PAs.
• Senecio species (e.g., ragwort): Common weeds with high PA content, dangerous to livestock.
• Heliotropium species: Another genus known for significant PA levels.

Preparing and using comfrey safely for external relief

While internal use is strictly prohibited, comfrey’s long history as a topical remedy holds considerable merit, provided it is used correctly. Comfrey contains allantoin, which promotes cell proliferation and tissue repair, forming the basis for its long history of topical use. For centuries, people have prepared poultices, salves, and compresses from comfrey leaf to address sprains, bruises, and skin irritations. A suspected fracture needs a doctor, not a poultice. For instance, a traditional poultice might involve crushing 3 to 5 fresh leaves (perhaps with the broad side of a lightweight garden trowel if a mortar is unavailable) and applying them directly to an affected area for 20 to 30 minutes. This method has been valued for its ability to soothe discomfort.

When preparing comfrey for external use, ensure that the skin is unbroken. Applying comfrey to open wounds or abraded skin bypasses the skin barrier that makes topical use tolerable in the first place and lets pyrrolizidine alkaloids reach the bloodstream. Keep comfrey on intact skin only. A common method for making a comfrey salve involves infusing 1 cup of chopped dried comfrey leaf in 2 cups of olive oil for several weeks, then straining and mixing with beeswax. Use leaf rather than root: roots hold the highest pyrrolizidine alkaloid concentrations of any part of the plant. This creates a stable product that can be stored for up to one year at room temperature. Always perform a patch test on a small area of skin first, especially if you have sensitive skin, to ensure no adverse reaction occurs within 24 hours of initial application. Do not use comfrey at all (internally or topically) if you are pregnant or nursing, on children, or if you have any liver condition or take medication that stresses the liver.

common topical comfrey preparations

• Poultice: Crushed fresh leaves applied directly to the skin for 20-30 minutes.
• Compress: Cloth soaked in comfrey tea (made by steeping leaves in hot water) and applied.
• Salve/Ointment: Comfrey-infused oil mixed with beeswax, stored for up to one year.
• Liniment: Alcohol-based extract for rubbing into muscles or joints.
• Infused oil: Leaves steeped in a carrier oil for several weeks.

Growing comfrey responsibly in your home garden

Growing comfrey in your home garden is a straightforward process, as it is a resilient perennial herb that thrives in a variety of conditions, particularly in USDA zones 3 to 9. It prefers full sun to partial shade and well-drained, rich soil. Start comfrey from root cuttings of the sterile hybrid Symphytum x uplandicum ‘Bocking 14’, planting them 2 to 3 feet apart in spring after the last frost. Common comfrey, S. officinale, sets viable seed and self-sows into places you did not choose. Once established, a single plant can spread up to 2 feet wide and reach heights of 3 feet, producing abundant foliage for your garden needs. Its vigorous growth means it can quickly become a dominant plant, so consider a dedicated bed or a location where its spread can be easily managed, perhaps with a root barrier extending 18 inches deep.

Regular harvesting manages comfrey's growth and maximizes leaf production. You can typically cut the leaves back to 4 to 6 inches above the ground three to five times per growing season, especially in warmer regions like USDA zone 8. After each harvest, a light application of compost or a balanced organic fertilizer can encourage rapid regrowth, often within two to three weeks. While comfrey is generally pest-resistant, monitoring soil moisture is important, particularly during dry spells; a soil moisture meter can help ensure optimal conditions, aiming for a consistent moisture level of 60% to 70%. Remember, the goal is to cultivate comfrey for its external and garden uses, keeping its internal risks firmly in mind.

tips for growing vigorous comfrey

• Planting: Use root cuttings, spaced 2 to 3 feet apart, in spring.
• Sunlight: Prefers full sun but tolerates partial shade.
• Soil: Thrives in well-drained, rich soil with a pH between 6.0 and 7.0.
• Watering: Keep soil consistently moist, especially during dry periods.
• Harvesting: Cut back leaves 4 to 6 inches from the ground three to five times per season.

Comfrey’s historical uses versus modern safety advisories

Comfrey shifted from traditional folk medicine to a plant with strict usage warnings as toxicological evidence developed. For centuries, across Europe and North America, comfrey was a staple in home apothecaries. Its common name, “knitbone,” reflects its widespread use for mending broken bones, sprains, and wounds, a practice documented as far back as 1936. Early 20th-century agricultural texts, such as one from 1910, even discussed prickly comfrey (Symphytum asperum) as a valuable forage crop for livestock, suggesting a perceived safety that has since been thoroughly disproven for internal use by animals and humans alike. This historical context reveals a long period of use before the full risks were understood.

However, by the late 1970s and early 1980s, research began to unequivocally link comfrey’s pyrrolizidine alkaloids to liver toxicity in humans and animals. This led to a gradual shift in medical and regulatory perspectives. The culmination of this research was the 2001 advisory from the US FDA, which explicitly warned consumers against ingesting comfrey in any form, including teas, supplements, and even small amounts in food. This advisory effectively ended the era of internal comfrey use in responsible herbal practice within the United States. Oral comfrey products largely left the mainstream US supplement market after the advisory, though the FDA’s action was guidance to manufacturers rather than a ban, and comfrey products can still be found for sale.

key milestones in comfrey safety

• Ancient Times: Used widely in traditional medicine for external injuries.
• 1910: Prickly comfrey noted as a forage crop under early assumptions of safety.
• 1936: Documented use in poultices for sprains and broken bones.
• 1970s-1980s: Scientific research identifies pyrrolizidine alkaloids as liver toxins.
• 2001: US FDA issues advisory against all internal comfrey consumption.

Comfrey vs. other garden herbs for external use

Liver Risk: The FDA issued an advisory in 2001, recommending against internal comfrey use due to the risk of liver damage from pyrrolizidine alkaloids.

Growth Habit: Comfrey (Symphytum officinale) can grow up to 3 feet tall and spread 2 feet wide, thriving in USDA zones 3 to 9 with abundant foliage.

Frequently asked questions

What are pyrrolizidine alkaloids?

Pyrrolizidine alkaloids (PAs) are naturally occurring toxic compounds found in various plants, including comfrey. When ingested, they are metabolized in the liver into toxic pyrroles, which can block small veins and cause veno-occlusive disease, leading to severe liver damage. Studies have identified over 600 different PAs across more than 6,000 plant species.

Is comfrey safe to use on broken skin?

No, it is generally advised to avoid applying comfrey to broken or abraded skin. Broken skin removes the barrier that limits absorption, so alkaloids can reach the bloodstream. Always ensure the skin is intact before topical application for maximum safety.

How long can I use comfrey topically?

For topical applications, comfrey is generally considered safe for short-term use, typically for periods of up to four to six weeks. Prolonged use over many months or years is often discouraged due to potential cumulative absorption, although significant adverse effects from topical use are rare. Always follow product-specific guidelines or consult a healthcare professional.

Can comfrey be used as animal feed?

Historically, some comfrey species like prickly comfrey (Symphytum asperum) were used as animal forage, as noted in 1910. However, due to the pyrrolizidine alkaloids, internal consumption by animals can also lead to liver damage, particularly in livestock like horses and cattle. Most modern agricultural advice recommends against feeding comfrey to animals to avoid health risks.

What part of the comfrey plant contains PAs?

Pyrrolizidine alkaloids are present in all parts of the comfrey plant, including the leaves, stems, and flowers, but they are found in the highest concentrations in the roots. Young leaves tend to have slightly lower concentrations than mature leaves, but the risk remains for all plant material. Therefore, no part of the plant should be consumed internally.

How does comfrey benefit garden soil?

Comfrey significantly benefits garden soil by acting as a dynamic accumulator, drawing essential nutrients like potassium and calcium from up to 10 feet deep. Its leaves are nitrogen-rich and decompose rapidly in compost, enriching the soil.

References

1. comfrey, n. (2023). comfrey, n..
2. The efficacy and safety of comfrey (2000). The efficacy and safety of comfrey.
3. COMFREY (1936). COMFREY.
4. Symphytum officinale (2022). Symphytum officinale.
5. Prickly comfrey as a forage crop / (1910). Prickly comfrey as a forage crop /.
6. USDA Natural Resources Conservation Service (2024). USDA Natural Resources Conservation Service.