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Lugol's iodine, also known as Lugol's solution, first made in 1829, is a solution of elemental iodine and potassium iodide in water, named after the French physician J.G.A. Lugol. Lugol's iodine solution is often used as an antiseptic and disinfectant, for emergency disinfection of drinking water, and as a reagent for starch detection in routine laboratory and medical tests. These uses are possible since the solution is a source of effectively free elemental iodine, which is readily generated from the equilibration between elemental iodine molecules and triiodide ion in the solution.
It has been used more rarely to replenish iodine deficiency. However, pure potassium iodide, containing the relatively benign iodide ion without the more toxic elemental iodine, is strongly preferred for this purpose. Likewise, in the Chernobyl disaster some Lugol's solution was used as an emergency source of iodide to block radioactive iodine uptake, simply because it was widely available as a drinking water decontaminant, and pure potassium iodide without iodine (the preferred agent) was not available.
Lugol's solution is available in different potencies of 1%, 2%, or 5% Iodine. The 5% solution consists of 5% (wt/v) iodine (I2) and 10% (wt/v) potassium iodide (KI) mixed in distilled water and has a total iodine content of 130 mg/mL. Potassium iodide renders the elementary iodine soluble in water through the formation of the triiodide (I−
3) ion. It is not to be confused with tincture of iodine solutions, which consist of elemental iodine, and iodide salts dissolved in water and alcohol. Lugol's solution contains no alcohol.
Other names for Lugol's solution are I2KI (iodine-potassium iodide); Markodine, Strong solution (Systemic); and Aqueous Iodine Solution BCP.
Lugol's is obtained over the counter from drug stores or health food stores. This indicator, also called a stain, is used in many different fields.
Lugol's was often used in the treatment of gout.
It was also used at one time as a first line treatment for hyperthyroidism, as the administration of pharmacologic amounts of iodine leads to temporary inhibition of iodine organification in the thyroid gland, a phenomenon called the Wolff-Chaikoff effect. However it is not used to treat certain autoimmune causes of thyroid disease as iodine-induced blockade of iodine organification may result in hypothyroidism. They are not considered as a first line therapy because of possible induction of resistant hyperthyroidism but may be considered as an adjuvant therapy when used together with other hyperthyrodism medications.
Because of its wide availability as a drinking-water decontaminant, and high content of potassium iodide, emergency use of it was at first recommended to the Polish government in 1986, after the Chernobyl disaster to replace and block any intake of radioactive 131I, even though it was known to be a non-optimal agent, due to its somewhat toxic free-iodine content. Other sources state that pure potassium iodide solution in water (SSKI) was eventually used for most of the thyroid protection after this accident. There is "strong scientific evidence" for potassium iodide thyroid protection to help prevent thyroid cancer. Potassium iodide does not provide immediate protection but can be a component of a general strategy in a radiation emergency.
Historically, Lugol's iodine solution has been widely available and used for a number of health problems with some precautions. Lugol's is sometimes prescribed in a variety of alternative medical treatments.
Until 2007, in the United States of America, Lugol's solution was unregulated and available over the counter as a general reagent, an antiseptic, a preservative, or as a medicament for human or veterinary application.
However, effective August 1, 2007, the DEA now regulates Lugol's solution (and, in fact, all iodine solutions containing greater than 2.2% iodine) as a List I precursor because it may potentially be used in the illicit production of methamphetamine. However, transactions of up to one fluid ounce (30 ml) of Lugol's solution are exempt from this regulation. By contrast, Lugol's iodine solution is available over the counter in Canada and Mexico.
Because it contains free iodine, Lugol's solution at 2% or 5% concentration without dilution is irritating and destructive to mucosa, such as the lining of the esophagus and stomach. Doses of 10 mL of 5% solution have been reported to cause gastric lesions when used in endoscopy.  The lethal dose of free iodine for an adult human of 2 to 3 grams (2000-3000 mg) free iodine represents 40 to 60 mL (less than 2 fluid ounces) of 5% Lugol's solution.