A brown discolouration developing after starting a prescribed mouthwash is a recognised effect rather than a sign that something has gone wrong. The mechanism is tied to how the rinse works.
Substantivity is the property that causes it
Most rinses are cleared from the mouth within minutes, which limits how long any active ingredient can act on bacteria.
Some antiseptics carry a positive charge and bind to the negatively charged surfaces of teeth, mucosa and the protein film coating them.
Bound molecules are released slowly over subsequent hours, extending the antibacterial effect well beyond the rinse itself. That retention is the entire reason such products are effective.
The same binding attracts pigment
Once bound to the tooth surface, these molecules present sites that dietary pigments readily attach to.
Tea, coffee, red wine and similar sources therefore deposit more heavily on a treated surface than on an untreated one.
The resulting stain is typically brown, concentrated between teeth and along the gum margin, and it appears within days of starting rather than gradually. People who drink a lot of tea or coffee see it soonest.
It is a surface deposit, not a change in the tooth
The discolouration sits on the acquired film rather than within enamel, which places it firmly in the extrinsic category.
Professional cleaning and polishing generally remove it, and it does not recur once the product is stopped. The tooth underneath has not changed colour at any point.
This is quite different from intrinsic discolouration, which lies within tooth structure and does not respond to cleaning at all.
Why these rinses are prescribed anyway
Antiseptic rinses of this type are used for defined purposes, such as after certain surgical procedures or where mechanical cleaning is temporarily impossible.
They are generally intended for short courses rather than indefinite use, partly because of staining and partly because of effects on taste.
Duration and suitability are clinical decisions, and using such a product long term without supervision is not what it was designed for.
Other rinses cause different effects
High alcohol content can contribute to dryness in people already producing little saliva, which works against the outcome the rinse was intended to support.
Some rinses containing metal salts also stain, and certain iron-containing preparations produce dark deposits by a similar surface mechanism.
Any rinse producing staining, altered taste or soreness is worth raising with the prescribing dentist rather than simply stopping or continuing without discussion.