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Biguanide antimicrobial

Chlorhexidine

Chlorhexidine is a broad-spectrum biguanide antimicrobial with bactericidal activity against gram-positive and gram-negative organisms, anaerobes, and some fungi. It is a topical agent used for skin antisepsis, oral rinse, wound irrigation, and ophthalmic care. The key testable fact: chlorhexidine disrupts bacterial cell membranes by binding to anionic sites.

Practice questionsHigh-yield facts
Drug classBiguanide antimicrobial agent
MechanismDisrupts bacterial cell membrane; binds anionic sites
Main usesOral rinse, skin antisepsis, wound irrigation, eye care
Key adverse effectAllergic contact dermatitis, reversible brown staining
3 min readFor medical studentsUpdated 2026-10-15

Mechanism of action

Chlorhexidine is a cationic biguanide that disrupts bacterial cell membranes. It binds to anionic (negatively charged) sites on bacterial cell surfaces, causing leakage of intracellular contents and cell death. This mechanism is bactericidal at therapeutic concentrations. The drug is effective against gram-positive bacteria, gram-negative bacteria, anaerobes, and some fungi. It has minimal activity against bacterial spores and viruses. The antimicrobial effect is concentration-dependent and is potentiated in acidic environments.

High-yield for exams
  • Chlorhexidine mechanism: disrupts bacterial cell membrane by binding anionic sites—bactericidal, not bacteriostatic.
  • Substantivity: prolonged oral activity (≈12 hours) due to tissue binding—distinguishes it from other antiseptics.
  • Brown tooth staining is reversible, common with oral rinse use, caused by interaction with chromogens.
  • Absolute contraindication: perforated tympanum (ototoxicity risk) and body cavity irrigation (severe toxicity).
  • Anionic surfactants (soap, SLS) inactivate chlorhexidine—rinse between applications.
  • Broad spectrum: gram-positive, gram-negative, anaerobes, some fungi; minimal activity against spores/viruses.
  • Minimal systemic absorption topically; safe in pregnancy when used on skin/oral mucosa per current data.
  • Preferred over povidone-iodine in many surgical settings; superior efficacy and lower sensitization risk.

Indications

  • Oral care: periodontal disease, gingivitis, aphthous ulcers, oral rinse pre- and post-operative
  • Skin antisepsis: surgical site preparation, skin disinfection, hand hygiene
  • Wound irrigation: clean and contaminated wounds, burn care
  • Ophthalmic: conjunctivitis, corneal ulcers, contact lens care (some formulations)
  • Genital and urinary tract: urinary catheter care, antiseptic for genital procedures
  • Otic: external ear antisepsis

Clinically, chlorhexidine is preferred over povidone-iodine in many settings due to superior efficacy and longer duration of action.

Pharmacokinetics

Chlorhexidine is a topical agent with minimal systemic absorption when applied to intact skin or mucous membranes. Following oral rinse or dental use, some absorption occurs through damaged mucosa. Absorbed drug is poorly metabolized and primarily excreted unchanged in urine. Protein binding is high (approximately 90%). The drug has a prolonged duration of action due to substantivity—the ability to bind to oral tissues and be slowly released over time, providing residual antimicrobial activity for up to 12 hours after application.

Adverse effects

Common:

  • Brown or yellow staining of teeth and oral tissues (reversible with cessation; due to interaction with dietary chromogens)
  • Taste alteration or dysgeusia
  • Increased calculus and tartar formation
  • Mild oral irritation or ulceration

Skin and allergy:

  • Allergic contact dermatitis (rare but important)
  • Irritation at application site
  • Cross-reactivity with other biguanides or related compounds

Serious (rare):

  • Anaphylaxis (uncommon; usually from preserved formulations)
  • Serious hypersensitivity reactions
  • CNS effects (confusion, ataxia) if inadvertently ingested in large amounts

Special considerations:

  • Ototoxicity reported with ear canal irrigation in cases of perforated tympanum
  • Corneal damage if instilled into the eye at concentrations above 0.12%

Contraindications and warnings

Absolute contraindications:

  • Perforated tympanum or middle ear disease (risk of ototoxicity)
  • Known hypersensitivity to chlorhexidine or other biguanides
  • Use in body cavities (e.g., peritoneal lavage) due to toxicity risk

Cautions:

  • Pregnancy and lactation: Limited data; use only when clearly needed. Minimal systemic absorption with topical use suggests low risk, but standard practice favors conservative use.
  • Deep wounds or abraded skin: Use dilute solutions and ensure adequate rinsing to minimize risk.
  • Black box warning or special alert: None standard, but inadvertent use in body cavity irrigation has caused severe adverse effects.
  • Patients with iodine allergy generally tolerate chlorhexidine well, but skin sensitivity varies.
  • Do not mix with anionic surfactants or soaps, which may reduce efficacy.

Drug interactions

Reduced efficacy:

  • Anionic surfactants (sodium lauryl sulfate, soap): Inactivate chlorhexidine; avoid concurrent use or rinse thoroughly between applications.
  • Mucins and pus: May reduce efficacy in infected or protein-rich environments.

No significant pharmacokinetic interactions with systemic drugs due to minimal absorption. However, topical application in the presence of alcohol-based rinses may increase local irritation. Chlorhexidine is compatible with most topical antibiotics and antiseptics when used sequentially with adequate rinsing.

Clinical pearls

  • Substantivity is a key exam feature: Chlorhexidine's prolonged oral activity (up to 12 hours) is due to tissue binding—higher than other oral antiseptics.
  • Staining pattern: Brown discoloration typically affects anterior teeth and gingiva; reversible upon discontinuation but may take weeks to resolve completely.
  • Concentration matters: Oral rinses typically 0.12%; higher concentrations (e.g., 4% for skin prep) increase irritation risk.
  • Superior to iodine in many settings: Chlorhexidine has broader spectrum, longer duration, and lower allergic sensitization than povidone-iodine.
  • Dilution for safety: Always dilute appropriately for wound irrigation; undiluted 4% solutions can cause irritation or toxicity in deep or body cavity wounds.
  • Preference in cardiac surgery: Chlorhexidine skin prep reduces surgical site infections more effectively than iodine-based agents in some guidelines.
  • Not a substitute for brushing: While useful adjunctively in gingivitis, mechanical plaque removal remains first-line in periodontal disease.
  • Cross-reactivity uncommon but possible: Patients sensitized to one biguanide (e.g., alexidine) may react to chlorhexidine; always obtain detailed allergy history.
Mnemonic

CHX = Cell membrane disruption, Hydrochloride/acetate/gluconate salt forms, eXcellent substantivity

Practice questions

1. A 45-year-old woman presents with acute gingivitis and periodontal disease. Her dentist prescribes chlorhexidine oral rinse 0.12% twice daily. Which of the following best explains the prolonged antimicrobial activity of chlorhexidine in the oral cavity?

Answer: B. Chlorhexidine exhibits substantivity, meaning it binds to oral tissues and is slowly released, providing residual antimicrobial activity for up to 12 hours. This tissue-binding property, not hepatic metabolism or RNA inhibition, explains its prolonged efficacy. Plasma protein binding does not contribute significantly to oral activity since chlorhexidine is a topical agent with minimal systemic absorption.

2. A 60-year-old man undergoes knee surgery and receives a chlorhexidine skin prep before incision. Two days post-op, the surgeon notes a mildly erythematous rash at the surgical site. Which drug interaction or application factor is most likely responsible for this complication?

Answer: C. Anionic surfactants (soap, sodium lauryl sulfate) inactivate chlorhexidine and may increase local irritation through incompatibility. If chlorhexidine is applied immediately after soap-based prepping without adequate rinsing, residual soap can both reduce efficacy and cause irritation. Prior iodine use and corticosteroids are less likely; systemic absorption is minimal; and true hypersensitivity would present differently (urticaria, angioedema).

3. A 72-year-old man with a history of repeated otitis externa is prescribed chlorhexidine ear drops for infection prevention. At a follow-up visit, he reports sudden hearing loss in the affected ear. Which aspect of chlorhexidine's safety profile is most directly relevant to this adverse event?

Answer: C. Chlorhexidine is contraindicated in perforated tympanum or middle ear disease due to risk of ototoxicity from direct inner ear exposure. If the tympanum is perforated, chlorhexidine can enter the middle and inner ear, causing hearing loss. This is a classic black-box-level caution. Hepatotoxicity and systemic accumulation are not typical issues; aminoglycoside cross-reactivity is unrelated; and binding is reversible in most tissues.

Frequently asked questions

How does chlorhexidine work?

Chlorhexidine is a cationic biguanide that binds to negatively charged sites on bacterial cell membranes, disrupting membrane integrity and causing leakage of intracellular contents. This mechanism is bactericidal and effective against a broad spectrum of gram-positive and gram-negative organisms, anaerobes, and some fungi.

What are the main side effects of chlorhexidine?

Common side effects include reversible brown staining of teeth and tissues, taste alteration, and increased calculus formation. Rare but serious effects include allergic contact dermatitis and anaphylaxis. Ototoxicity and corneal damage can occur if used in the ear (perforated tympanum) or eye (high concentrations) respectively.

Is chlorhexidine safe in pregnancy?

Chlorhexidine has minimal systemic absorption when applied topically to skin or oral mucosa, suggesting low risk in pregnancy. Current evidence supports use when clearly needed, though conservative practice favors avoiding routine use. The benefit-risk assessment should guide clinical decision-making.

Chlorhexidine versus povidone-iodine—what is the difference?

Chlorhexidine has a broader spectrum of activity, longer duration of action (substantivity up to 12 hours), and lower allergic sensitization rates compared to povidone-iodine. Chlorhexidine is preferred in most surgical and oral settings. Iodine-based agents are useful in iodine-allergic patients but are less commonly used today.

Why does chlorhexidine cause tooth staining?

Brown discoloration results from interaction between chlorhexidine (cationic) and dietary chromogens (anionic compounds in food and beverages). This staining is reversible and typically resolves within weeks after discontinuation. It is a cosmetic concern but does not indicate toxicity and is more common with long-term oral rinse use.

Conditions it is used for

GingivitisPeriodontitisAphthous ulcersSurgical site infection (prophylaxis)Wound infection (topical adjunct)ConjunctivitisCorneal ulcerExternal otitis
Povidone-iodineHexachloropheneTriclosanBenzalkonium chlorideChloroxylenolOctenidineIodine solutionAlcohol-based hand sanitizer

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: A01AB03WHO ATC/DDD index: B05CA02

Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.