Chlorhexidine is the most popular antiseptic of this group. 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 sec. However, it is relatively ineffective against other gram-negative organisms, spores, fungi, and most viruses. Nosocomial infections by Pseudomonas spp have developed from the use of contaminated chlorhexidine solutions in which the bacteria persisted. In susceptible organisms, chlorhexidine disrupts the cytoplasmic membrane. Its activity is 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. It is incompatible with anionic compounds, including soap.
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% (w/v) solution in 70% isopropanol as a general antiseptic, and a 0.5% solution in 70% isopropanol with emollients as a hand rinse. Chlorhexidine soaps have good residual activity, which 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 in treatment of external wounds in dogs, cats, and horses. Contact dermatitis has been reported in up to 8% of human patients after repeated topical exposure. Little data are available on hypersensitivity reactions in animals.