What is Boric Acid?
Boric acid is a weak Lewis acid (pKa 8.92-9.24) that functions as a bacteriostatic preservative in laboratory specimens and as a topical antimicrobial agent, particularly effective against Candida species in vaginal infections and Aspergillus in otomycosis. 1
Chemical Properties and Mechanism
- Boric acid exists at physiological pH as a molecular compound (H3BO3) that inhibits NAD-dependent enzymes in microorganisms 2
- It causes mitochondrial failure in Candida species, increases ethanol production from glucose, and decreases glycogen storage 2
- The compound is completely absorbed orally with a biological half-life of approximately 21 hours in humans 1
- It is naturally found in seawater and released from minerals through weathering processes 3
Clinical Applications
Vaginal Candidiasis Treatment
- The Infectious Diseases Society of America recommends intravaginal boric acid 600 mg daily for 14 days as first-line therapy for recurrent vaginal Candida glabrata infection, achieving approximately 70% eradication rates even in azole-unresponsive cases 4
- FDA-approved formulations are available as vaginal suppositories for quick relief of yeast infections and bacterial vaginosis symptoms 5
- Boric acid works by restoring normal vaginal pH and has been previously available only by prescription 5
Aspergillus Otomycosis
- The Infectious Diseases Society of America recommends topical irrigating solutions of boric acid (along with acetic acid or azole cream) as effective treatment for eradicating Aspergillus otomycosis 6
- This application targets the external auditory canal where Aspergillus species (primarily A. niger and A. fumigatus) cause saprophytic infections 6
Laboratory Specimen Preservation
- Boric acid functions as a bacteriostatic transport medium for urine specimens, preserving bacterial colony counts and organism identification for up to 48 hours 6
- It prevents overgrowth of bacteria during transport delays, maintaining diagnostic accuracy of urine cultures 6
- The cost-effectiveness is notable at approximately 0.07 pence per specimen 6
Resistance Profile
- Evolutionary studies demonstrate significant constraints on the development of boric acid resistance and tolerance in Candida albicans 7
- After 11 weeks of experimental evolution under increasing boric acid concentrations, populations showed only modest growth improvements at low drug levels with no large increases in resistance 7
- This evolutionary constraint explains why boric acid remains effective for recurrent yeast infections despite decades of use 7
Safety Considerations
- Boric acid is well-tolerated topically but high systemic exposure can cause toxicity, including death, particularly in children 1
- Absorption through intact skin is negligible, though it can occur through denuded or irritated skin 3
- Animal studies have noted toxicities including reductions in male sperm counts at high exposures 1
- The compound does not persist in the body upon cessation of exposure 3