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Potassium bromide (KBr) is a salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries, with over-the-counter use extending to 1975 in the US. Its action is due to the bromide ion (sodium bromide is equally effective). Potassium bromide is used as a veterinary drug, in antiepileptic medication for dogs. Under standard conditions, potassium bromide is a white crystalline powder. It is freely soluble in water; it is not soluble in acetonitrile. In a dilute aqueous solution, potassium bromide tastes sweet, at higher concentrations it tastes bitter, and tastes salty when the concentration is even higher. These effects are mainly due to the properties of the potassium ion—sodium bromide tastes salty at any concentration. In high concentration, potassium bromide strongly irritates the gastric mucous membrane, causing nausea and sometimes vomiting (a typical effect of all soluble potassium salts).^{[citation needed]}

Potassium bromide, a typical ionic salt, is fully dissociated and near pH 7 in aqueous solution. It serves as a source of bromide ions. This reaction is important for the manufacture of silver bromide for photographic film:

KBr_(aq) + AgNO_3(aq) → AgBr_(s) + KNO_3(aq)

Aqueous bromide Br⁻ also forms complexes when reacted with some metal halides such as copper(II) bromide:

2 KBr_(aq) + CuBr_2(aq) → K₂[CuBr₄]_(aq)

A traditional method for the manufacture of KBr is the reaction of potassium carbonate with an iron(III, II) bromide, Fe₃Br₈, made by treating scrap iron under water with excess bromine:^[4]

4 K₂CO₃ + Fe₃Br₈ → 8 KBr + Fe₃O₄ + 4 CO₂

The anticonvulsant properties of potassium bromide were first noted by Sir Charles Locock at a meeting of the Royal Medical and Chirurgical Society in 1857. Bromide can be regarded as the first effective medication for epilepsy. At the time, it was commonly thought that epilepsy was caused by masturbation.^[6] Locock noted that bromide calmed sexual excitement and thought this was responsible for his success in treating seizures. In the latter half of the 19th century, potassium bromide was used for the calming of seizure and nervous disorders on an enormous scale, with the use by single hospitals being as much as several tons a year (the dose for a given person being a few grams per day).^[6] By the beginning of the 20th century, the generic word had become so widely associated with being sedate that the term ‘bromide’ came to mean a dull, sedate person or a boring platitude uttered by such a person.^[7]

There was not a better epilepsy drug until phenobarbital in 1912. The British Army has historically been claimed to lace soldiers’ tea with bromide to quell sexual arousal and in the Victorian era prisoners in England were compulsorily dosed with the chemical.^[8][9]

Bromide compounds, especially sodium bromide, remained in over-the-counter sedatives and headache remedies (such as the original formulation of Bromo-Seltzer) in the US until 1975, when bromides were outlawed in all over-the-counter medicines, due to chronic toxicity.^[10] Bromide’s exceedingly long biological half-life made it difficult to dose without side effects. Medical use of bromides for humans in the US was discontinued at this time, as many better and shorter-acting sedatives were known by then.