Mouthwash
Mouthwash or mouth rinse is a product used for oral hygiene.  Antiseptic and anti-plaque mouth rinse claims to kill the bacterial plaque  causes caries, gingivitis, and bad breath.  Anti-cavity mouth rinse uses fluoride to protect against tooth decay.  However, it is generally agreed that the use of mouthwash does not eliminate the need for both brushing and flossing.  In the absence of a ready-made mouthwash, gargling with plain water is preferable, to remove food particles, sugars and other pollutants in the mouth.
Mouth washes may also be used to help remove mucous and food particles deeper down in the throat.  Alcoholic and strong flavored mouth washes may cause coughing for this purpose.

# History
The first  known reference to mouth rinsing is in the Chinese medicine, about 2700 BCE, for treatment of gingivitis. Later, in the Greek and Roman periods, mouthrinsing following mechanical cleansing became common among the upper classes, and Hippocrates recommended a mixture of salt, alum and vinegar. The Jewish Talmud, dating back about 1800 years, suggests a cure for gum ailments containing "dough water" and olive oil.
Anton van Leeuwenhoek, the famous 17th century microscopist, discovered living organisms (living, because they were motile) in deposits on the teeth (what we now call dental plaque). He also found organisms in water from the canal next to his home in Delft. He experimented with samples by adding vinegar or brandy and found that this resulted in the immediate immobilization or killing of the organisms suspended in water. Next he tried rinsing the mouth of himself and somebody else with a rather foul mouthwash containing vinegar or brandy and found that living organisms remained in the dental plaque. He concluded  correctly  that the mouthwash either did not reach, or was not present long enough, to kill the plaque organisms.
That remained the state of affairs until the late 1960s when Harald Loe (at the time a professor at the Royal Dental College in Aarhus, Denmark) demonstrated that a chlorhexidine compound could prevent the build-up of dental plaque. The reason for chlorhexidine effectiveness is that it strongly adheres to surfaces in the mouth and thus remains present in effective concentrations for many hours.
Since then commercial interest in mouthwashes has been intense and several newer products claim effectiveness in reducing the build-up in dental plaque and the associated severity of gingivitis (inflammation of the gums), in addition to fighting bad breath. Many of these solutions aim to control the Volatile Sulfur Compound (VSC)-creating anaerobic bacteria that live in the mouth and excrete substances that lead to bad breath and unpleasant mouth taste.

# Usage
Common use involves rinsing the mouth with about 20ml (2/3 fl oz) of mouthwash two times a day after brushing.  The wash is typically swished or gargled for about half a minute and then spat out. In some brands, the expectorate is stained, so that one can see the bacteria and debris. However it is probably advisable to use mouthwash at least an hour after brushing with toothpaste, since the anionic compounds in the toothpaste can inactivate cationic agents present in the mouthrinse. Probably the most effective time to rinse and gargle with a mouthrinse is at bed time.

# Composition
Active ingredients in commercial brands of mouthwash can include thymol, eucalyptol, hexetidine, methyl salicylate, menthol, chlorhexidine gluconate, benzalkonium chloride, cetylpyridinium chloride, methylparaben, hydrogen peroxide, domiphen bromide and sometimes fluoride, enzymes and calcium. Ingredients also include water, sweeteners such as sorbitol, Sucralose, sodium saccharine, and xylitol (which doubles as a bacterial inhibitor).
Sometimes a significant amount of alcohol (up to around 20%) is added, as a carrier for the flavor, to provide "bite" and to contribute an antibacterial effect.  Because of the alcohol content, it is possible to fail a breathalyzer test after rinsing; in addition, alcohol is a drying agent and may worsen chronic bad breath. As such, it is possible for alcoholics to abuse mouthwash. Recently, some assumptions were made of a possible carcinogenic character of alcohol used in mouthrinses, but no clear evidence was found. Commercial mouthwashes usually contain a preservative such as sodium benzoate to preserve freshness once the container has been opened. Many newer brands are alcohol-free and contain odor-elimination agents such as oxidizers, as well as odor-preventing agents such as zinc ion technology to keep future bad breath from developing.
A salt mouthwash is a home treatment for mouth infections and/or injuries, or post extraction, and is made by dissolving a teaspoon of salt in a cup of warm water.  Plain (diluted) hydrogen peroxide is another common mouthwash.
One thing to note is that many commercial mouthwashes are very acidic on the pH scale. If you have heartburn, acid reflux or acid indigestion, it is important to use a mouthwash with a neutral pH to avoid irritation..