Chlorhexidine is the most popular antiseptic of the biguanides. It has potent antimicrobial activity against most gram-positive and some gram-negative bacteria but not against spores. A 0.1% aqueous solution is bactericidal against Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa in 15 seconds. However, against other gram-negative organisms, spores, fungi, and most viruses, chlorhexidine is relatively ineffective. Importantly, nosocomial infections due to Pseudomonas-contaminated chlorhexidine solutions have been reported. In susceptible organisms, chlorhexidine disrupts cytoplasmic membranes. Its activity is either unaffected or enhanced by alcohols, quaternary ammonium compounds, and alkaline pH, and is somewhat depressed by high concentrations of organic matter (pus, blood, etc), hard water, and contact with cork. Chlorhexidine is cationic and is therefore incompatible with anionic compounds, including most soaps.
Chlorhexidine is one of the most commonly used surgical and dental antiseptics. A 4% emulsion of chlorhexidine gluconate is used as a skin cleanser; a 0.5% solution in 70% isopropanol, as a general antiseptic; and a 0.5% solution in 70% isopropanol with emollients, as a hand rinse. Though incompatible with traditional soaps, chlorhexidine scrub is itself a detergent; it has good residual activity and may be advantageous when applied as a presurgical scrub for prolonged surgical procedures.
Chlorhexidine-alcohol mixtures are particularly effective in that they combine the antiseptic rapidity of alcohol with the persistence of chlorhexidine. Because of its antiseptic properties and low potential for systemic or dermal toxicity, chlorhexidine has been incorporated into shampoos, ointments, skin and wound cleansers, teat dips, and surgical scrubs. A 1% chlorhexidine acetate ointment is used as a topical antiseptic to treat external wounds in dogs, cats, and horses. While an allergic response is always possible with any topically applied agent, chlorhexidine is not associated with contact dermatitis in animals; it is commonly used as an alternative for those animals known to experience contact dermatitis after iodophor application.
If the concentration of chlorhexidine used is too low (<4% ), resistance to both chlorhexidine and certain antimicrobials can result. Contamination of chlorhexidine has been linked to multiple nosocomial outbreaks of infection. In most of these cases, contaminated water was used to dilute solutions and/or bottles were not adequately disinfected before being used to dispense chlorhexidine. Most of these outbreaks were associated with use of solutions that contained <2% chlorhexidine.
Chlorhexidine is ototoxic and should not be used to clean ears, nor should it be applied to the eye.