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Comfrey oil is a natural plant-based oil derived from the comfrey plant, a widely-cultivated perennial herb that has been grown from at least the ancient Roman and Greek period for its medicinal properties. The oil can be derived from several related species of comfrey plant, including Symphytum officinale and Symphytum asperum, but the most commonly used variety as of 2011 is a hybrid plant based on the parent plants of these two strains, known as Symphytum uplandicum. While the comfrey herb can be used in its natural state, it is usually steeped in a hot liquid to extract its essential qualities. It is, therefore, often an ingredient in other beneficial oils such as olive oil or almond oil, where these mixtures are referred to as macerated oils due to the addition of beneficial herbs.
Traditional medicinal uses for comfrey have centered around making the plant into creams and teas as it is considered to be a healing salve and ingredient for treating bone fractures or damage to the skin. Other benefits of comfrey oil include that it can relieve pain from degenerative conditions such as arthritis and osteoarthritis, as well as irritation from insect bites. Comfrey cream does this by the presence of a compound called allantoin which has demonstrated an ability to stimulate cell production in the body. Comfrey oil is also often mixed with antibacterial compounds so that it promotes the rapid healing of damage to the skin while bacteria are not trapped under the surface where they can continue to grow.
Among the chief drawback of using comfrey oil is the potential for it to have carcinogenic effects on the human liver in large quantities as shown in lab research during the 1970s. The side effects of comfrey are based on the pyrrolizidine alkaloid compounds that it contains in its leaves, though these carcinogenic effects have only been confirmed in rats who were fed a diet that was composed of 33% comfrey leaves alone. Research involving the use of the entire plant has shown counter indications of benefits to the liver, and macerated vinegar based on the whole comfrey plant is often used in Japan to treat cirrhosis. Estimates are that, in order for a human being to simulate the conditions of diet for the rats in the research, an average person would have to consume three to four cups (700 to 950 mL) of comfrey oil based tea on a daily basis for 140 years.
Since comfrey oil has the potential for negative side effects, many natural medicine practitioners recommend that its use be confined to comfrey cream and lotions that are applied topically. It has the potential to cause allergic reactions in some individuals as well and long-term use is not recommended. Despite these findings, however, comfrey oil has been used for centuries as a native plant in Europe. The plant grows best in damp, grass covered fields so it is most commonly found in the wild in regions of Ireland and the UK such as along partially shaded river banks, though Symphytum uplandicum's common name is Russian Comfrey.