Menthol?

Toothpaste is composed mainly of soap and/or detergent mixed with powdered chalk (a mild abrasive) made into a slurry with water.

The flavoring used is normally double rectified oil of peppermint, which is the only thing strong enough to cover up the soap taste, and sweetened with saccharine.

When you brush your teeth it's pretty much pegging all the meters in the nerves in your tongue and overloading your taste buds with extreme stimulation. It takes them a little while to recover afterwards. Rinsing your mouth out with water after you brush your teeth will help this along.

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