Horsetail is best known as a diuretic, or substance that increases water loss from the body. As a topical medicine, horsetail has been used to treat open wounds and burns, and to stop bleeding. Horsetail has large amounts of the mineral silica deposited in its stems. Because of this, horsetails were used as a material for scouring pots and pans and in works of pewter. Horsetail is commonly found growing near water, or in locations where its extremely long taproots can find consistent sources of water.
Horsetail resembles a stalk of asparagus, with solitary shoots protruding from the ground. Only the portion of the plant that grows above ground is used for medicinal purposes. Some evidence validates its historical use as a diuretic; the plant is thought to have a mild diuretic effect.  The active constituents of this plant include flavone glycosides and equisetonin. Interestingly, small amounts of nicotine are found in this plant as well; however, no uses of the herb have been ascribed to this particular constituent. 
Horsetail is used as a diuretic in conditions of swelling; for kidney and bladder stones (to assist in flushing them out), urinary tract infections, and in the past, any condition involving the kidney or bladder. Horsetail was used extensively in the treatment of wounds, especially in children. Reasons for this are not exactly clear, however the plant withstood a great many number of years as a favorite tool for stopping both internal and external bleeding, as well as with speeding wound healing.
Other uses include; treatment of brittle fingernails, gout, frostbite, excessively heavy menstruation, and rheumatic disease.
As a compress for wound healing, horsetail is applied from a mixture containing approximately 10 grams of the dried plant in one liter of water.  In tincture form, one to four milliliters of the extract is recommended, three times per day.
These suggestions are taken from historical accounts. Relatively little modern research exists in which these doses have been validated.
Horsetail Side effects:
Long-term use of horsetail may lead to thiamin deficiency, a potentially serious condition in which certain neurologic functions are diminished. Because of this, horsetail products sold in Canada are required to be free of the constituent that leads to thiamine destruction in the herb. Children who chewed on several stems were observed to suffer from nicotine poisoning as a result of the nicotine content in the stems. Topical application of horsetail may cause seborrheic dermatitis, a type of mild skin inflammation that can be resistant to treatment.
Horsetail General interactions (supplement, herb, food, lab):
As mentioned earlier, horsetail may destroy thiamin in the body, so this should be a consideration in all types of diet, and those wishing to begin a supplementation regimen with this herb. Horsetail does not appear to affect any laboratory testing.
Taken with cardiac-specific herbs (those containing cardiac glycosides), horsetail may lead to glycoside toxicity as it assists in removal of potassium from the body.  Similarly, when horsetail is combined with laxative herbs, this interaction may lead to enhanced loss of potassium from the body. Potassium is a necessary electrolyte/ mineral required for multiple physiologic functions.
Horsetail has relatively large amounts of the mineral chromium as well. Taking the herb with chromium-containing supplements may predispose one to chromium toxicity.  Heavy use of horsetail while taking licorice may also lead to the enhanced loss of potassium from the body. 
Horsetail Drug interactions:
On a theoretical basis, when combined in large amounts with drugs such as corticosteroids, digoxin, and diuretic medicines, horsetail may potentiate potassium loss from the body, leading to various signs of hypokalemia. [6, 7] Potassium replacement should be a consideration in these circumstances.
Horsetail Disease conditions:
People with weakened kidney or heart function may be at greater risk of potassium depletion, from the subsequent increased excretion caused by this herb. In people with pre-existing thiamine deficiency, horsetail should be avoided.
1 Gruenwald J, Brendler T, Jaenicke C. PDR for Herbal Medicines. 1st ed. Montvale, NJ: Medical Economics Company, Inc., 1998.
2 The Review of Natural Products by Facts and Comparisons. St. Louis, MO: Wolters Kluwer Co., 1999.
3 Gruenwald J, Brendler T, Jaenicke C. PDR for Herbal Medicines. 1st ed. Montvale, NJ: Medical Economics Company, Inc., 1998.
4 McGuffin M, Hobbs C, Upton R, Goldberg A, eds. American Herbal Products Association’s Botanical Safety Handbook. Boca Raton, FL: CRC Press, LLC 1997. 2 Ibid
5 Brinker F. Herb Contraindications and Drug Interactions. Sandy, OR: Eclectic Medical Publ, 1997.
6 Lanca S, Alves A, Vieira AI, et al. Chromium-induced toxic hepatitis. Eur J Intern Med 2002;13:518-20. 5 Ibid
7 Newall CA, Anderson LA, Philpson JD. Herbal Medicine: A Guide for Healthcare Professionals. London, UK: The Pharmaceutical Press, 1996.
8 Robbers JE, Speedie MK, Tyler VE. Pharmacognosy and Pharmacobiotechnology. Baltimore, MD: Williams & Wilkins, 1996.