Toxic Properties:
Comfrey has limited value as food for humans even though the young leaves are considered edible. The leaf is hairy and the texture is mucilaginous, and the plant also has toxic properties. Internal usage of comfrey should be avoided because it contains hepatotoxic pyrrolizidine alkaloids (PAs). Use of comfrey can, because of these PAs, lead to veno-occlusive disease (VOD). VOD can in turn lead to liver failure, and comfrey, taken in extreme amounts, has been implicated in at least one death. Excessive doses of Symphytine, one of the PAs in comfrey, may cause cancer in rats. This was shown by injection of the pure alkaloid. The whole plant has also been shown to induce precancerous changes in transgenic rats.

Medicinal Uses:
Comfrey has been cultivated in China as a green vegetable and has been used as an herbal medicine for more than 2,000 years. Comfrey's original name, knitbone, comes from the external use of poultices of its leaves and roots to heal burns, sprains, swelling, and bruises. In western Europe, comfrey has been used topically for treating inflammatory disorders such as arthritis, gout, and thrombophlebitis, and internally for treating diarrhea. Comfrey has been claimed to heal gastric ulcers and hemorrhoids, and to suppress bronchial congestion and inflammation. In some countries comfrey distribution has been restricted because of the potential health hazard and toxicity of the plant if used internally.
Comfrey is used externally as a poultice, fomentation or liniment to speed the healing of wounded tissue, broken bones, and sprains. It is traditionally used internally as an anti-inflammatory and to soothe and heal gastro-intestinal tract tissue. Comfrey is recommended for external use only. The therapeutic use of comfrey is limited because of its potential toxicity. Comfrey has antifungal and anticancer activity, and has been used topically for treating musculoskeletal and inflammatory disorders. Because of limited evidence and toxicity, comfrey is not recommended for oral use. Traditionally comfrey has been used for ulcers, fractures, bruises, diarrhea, cough, bronchitis, and gum disease. Comfrey leaves are considered being of high value as an external remedy. The whole plant has been considered excellent for soothing pain in any tender or inflamed body part. It was formerly applied to raw, indolent ulcers as a glutinous astringent. It is useful in any kind of inflammatory swelling.

Other Uses:
Comfrey is a fast growing plant, producing large amounts of leaf during the growing season. Mature comfrey plants can be harvested up to four or five times a year. They are ready for cutting when about 2 feet high. Comfrey will rapidly regrow, and will be ready for further cutting about 5 weeks later. There are various ways in which comfrey can be utilized as a fertilizer. Comfrey for potatoes: Freshly cut comfrey should be wilted for a day or two, then laid along potato trenches about 2 inches deep. Avoid using flowering stems as these can root. The leaves will rapidly break down and supply potassium rich fertilizer for the developing potato plants. Comfrey as a compost activator: Include 2-3 inch deep layers of comfrey in the compost heap to encourage bacterial activity and help to heat the heap. Comfrey should not be added in quantity as it will quickly break down into a dark sludgey liquid that needs to be balanced with more fibrous, carbon rich material.
Comfrey liquid fertilizer: It can be produced by either rotting leaves down in rainwater for 4-5 weeks to produce a ready to use 'comfrey tea', or by stacking dry leaves under a weight in a container with a hole in the base. When the leaves decompose a thick black comfrey concentrate is collected. This must be diluted at 15:1 before use. Comfrey as a mulch: A 2 inch layer of comfrey leaves placed around a crop will slowly break down and release plant nutrients. It is especially useful for crops that need extra potassium, such as tomatoes, and also fruit bushes like gooseberries and currants.