Melzers reagent is an aqueous solution of chloral hydrate, potassium iodide, and iodine. Depending on the formulation, it consists of approximately 2.50-3.75% potassium iodide and 0.75–1.25% iodine, with the remainder of the solution being 50% water and 50% chloral hydrate. Melzers is toxic to humans if ingested due to the presence of iodine and chloral hydrate. Due to the legal status of chloral hydrate, Melzer's reagent is difficult to obtain in the United States.
In response to difficulties obtaining chloral hydrate, scientists at Rutgers formulated Visikol (compatible with Lugol's iodine) as a replacement. In 2019, research showed that Visikol behaves differently to Melzer’s reagent in several key situations, noting it should not be recommended as a viable substitute.
Melzers reagent is part of a class of iodine/potassium iodide (IKI)-containing reagents used in biology; Lugols iodine is another such formula.