Chlorhexidine: effective, and precisely for that reason not for long-term use

Chlorhexidine is the most studied oral antiseptic and the most overused, because it works well and that leads to prescribing it as though it had no consequences.

The mechanism explains both the effectiveness and the limits. The molecule is positively charged and binds to the negatively charged surfaces of the mouth: mucosa, dental pellicle and bacterial membranes.

That binding produces substantivity, the ability to remain active for hours after rinsing as it is gradually released. It is the property that sets it apart from other antiseptics.

The concentrations are not interchangeable. 0.2% is the concentration for short-term use in acute phases; 0.12% offers comparable efficacy with fewer side effects; lower concentrations are used over longer periods.

The best-known side effect is brown staining of teeth, restorations and tongue. It is not a deposit of the drug but the result of a reaction with dietary compounds, above all the polyphenols in coffee, tea and red wine.

This explains why staining varies so much between patients: it depends on what they drink. A patient who drinks a lot of coffee stains within days, one who drinks none barely at all.

The other effects are temporary taste alteration and, with prolonged use, desquamation of the mucosa. They reverse on stopping but are enough to make patients abandon treatment.

Hence the practical rule: chlorhexidine is a medicine for defined periods, not a daily mouthwash. Typical indications fall within one or two weeks.

The situations in which it is genuinely indicated are few and recognisable. The post-operative period, when brushing is not possible; acute necrotising forms; patients temporarily unable to perform mechanical hygiene.

Outside these cases the daily mouthwash should be something else, because a patient using chlorhexidine for months accumulates staining that then requires an appointment to remove.

One practical detail changes the outcome: chlorhexidine is inactivated by the sodium lauryl sulphate present in many toothpastes. Using it straight after brushing reduces its efficacy.

The correct advice is to leave at least half an hour between brushing and rinsing, and it is information almost no patient receives.

In summary: substantivity makes it effective for hours, the concentrations have different indications, staining comes from a reaction with dietary polyphenols, use should be limited to one or two weeks, and it must be kept apart from the toothpaste that inactivates it.