preservative
A preservative is the small dose of microbe-fighting chemical added to a medicine to stop it going off — the pharmaceutical equivalent of what keeps an opened jar of sauce from growing mould in the fridge. It does not treat the patient; it protects the product.
Formally, a preservative is an excipient added to inhibit the growth of bacteria, yeasts and moulds in multidose liquid and semisolid products. Water is what microbes need, so any preparation with a water phase that will be opened and re-used — eye drops, oral solutions, suspensions, creams, gels — generally requires one. Common examples include the parabens, benzalkonium chloride, benzoic acid and phenoxyethanol, each effective over a particular pH range.
Effectiveness must be balanced against safety. A preservative has to be active at a low, non-irritant concentration, stable, and compatible with the rest of the formula, yet many can cause sensitisation or sting sensitive tissues such as the eye. Because some preservatives partition into oil or bind to packaging, the formulator must ensure enough free preservative remains in the water phase to actually work — verified by a preservative efficacy (challenge) test.
Benzalkonium chloride preserves many multidose eye drops, but because it can irritate the ocular surface with chronic use, single-dose preservative-free units are preferred for patients on long-term glaucoma therapy.
A small amount of antimicrobial protects a re-opened product without treating the patient.
Only the preservative dissolved free in the water phase is active; if it partitions into oil, binds to the container or is neutralised by pH, the product can fail a challenge test despite the label dose being present.