Cetylpyridinium chloride

Cetylpyridinium chloride (CPC) is a cationic quaternary ammonium compound used in some types of mouthwashes, toothpastes, lozenges, throat sprays, breath sprays, and nasal sprays. It is an antiseptic that kills bacteria and other microorganisms. It has been shown to be effective in preventing dental plaque and reducing gingivitis. It has also been used as an ingredient in certain pesticides. 
Though one study seems to indicate cetylpyridinium chloride does not cause brown tooth stains, at least one mouthwash containing CPC as an active ingredient bears the warning label "In some cases, antimicrobial rinses may cause surface staining to teeth," following a class-action lawsuit brought by customers whose teeth were stained.
The name breaks down as:
Medical use.
OTC (Over The Counter) products containing cetylpyridinium chloride include oral wash, oral rinse, as well as ingestable products (e.g. CPC lozenges). In addition, cough syrups containing CPC are also available as OTC medications.
The FDA monograph on oral antiseptic drug products reviewed the data regarding CPC and concluded the following:
In addition, the National Library of Medicine Toxicology Data Network (TOXNET) reviewed the range of toxicity of CPC, and stated that, "Significant toxicity is rare after exposure to low concentration products that are typically available in the home."
The fatal dose in humans ingesting cationic detergents has been estimated to be 1 to 3 g. Therefore, a person using a typical oral ingestible product that provides 0.25 mg CPC per dose would need to take 4000 doses at one time before they would be in the estimated fatal dose range.
A review found that mouthwashes containing CPC "provide a small but significant additional benefit when compared with toothbrushing only or toothbrushing followed by a placebo rinse" in reducing plaque and gingivitis-inflammation. In combination with chlorhexidine and zinc lactate, CPC has been found to be effective in treating halitosis.
Side effects.
Tooth staining.
Cetylpyridinium chloride is known to cause tooth staining in approximately 3 percent of users. The Crest brand has noted that this staining is actually an indication that the product is working as intended, as the stains are a result of bacteria dying on the teeth. Crest stated that because of the low incidence of staining, there was no need to label Pro-Health mouthwash as a potential tooth stainer. However, after numerous complaints and a federal class-action lawsuit, which was later dismissed, the mouthwash now contains a label warning consumers of its potential to stain teeth.
Toxicology and pharmacology.
The of cetylpyridinium chloride has been measured at 30 mg/kg in rats and 36 mg/kg in rabbits when the chemical is administered by intravenous infusion but 200 mg/kg in rats, 400 mg/kg in rabbits, and 108 mg/kg in mice when administered orally.
Chemistry.
The molecular formula of cetylpyridinium chloride is C21H38NCl. In its pure form it is a solid at room temperature. It has a melting point of 77 °C when anhydrous or 80–83 °C as a monohydrate . It is soluble in water but insoluble in acetone, acetic acid, or ethanol. It has a pyridine-like odor. It is combustible. Concentrated solutions are destructive to mucous membranes. Its critical micelle concentration (CMC) is ~ 0.0009-0.0011M, and is strongly dependent on the salt concentration of the solution.
Some products are formulated instead with the bromide salt cetylpyridinium bromide, the properties of which are virtually identical.