Vancomycin is an antibiotic used to treat serious bacterial infections, but its administration route dictates its purpose. The intravenous (IV) form is used to treat systemic infections, such as bloodstream infections or pneumonia. In contrast, the oral formulation treats infections confined to the intestinal tract, primarily Clostridioides difficile infection (CDI), which causes severe diarrhea and colitis. This oral medicine works by acting directly on the target bacteria within the gut lumen, as it is not absorbed into the bloodstream in significant amounts. This localized action is the defining characteristic that separates oral vancomycin from most other oral antibiotics.
How Oral Vancomycin Kills Bacteria
Oral vancomycin functions as a glycopeptide antibiotic, which means it interferes with the production of the bacterial cell wall. This mechanism is highly specific to Gram-positive bacteria, such as C. difficile. The vancomycin molecule binds tightly to a specific pair of amino acid building blocks, D-alanyl-D-alanine, found at the end of the cell wall precursor units.
By binding to this specific site, vancomycin prevents the enzyme transpeptidase from linking these precursor units together to complete the peptidoglycan structure of the cell wall. The resulting incomplete cell wall is structurally compromised, ultimately leading to the destruction and death of the C. difficile bacteria localized within the colon. Since this action is entirely localized, the drug is highly effective at reducing the C. difficile population directly at the site of infection.
Why Oral Vancomycin Stays in the Gut
The poor systemic absorption of oral vancomycin is determined by its physical and chemical properties. Vancomycin is a large, complex glycopeptide molecule that the intestinal lining cannot easily transport across the gut wall and into the bloodstream. As a result, the drug remains within the gastrointestinal tract, where it achieves very high concentrations.
This minimal absorption is why oral vancomycin is ineffective against infections elsewhere in the body and is the treatment of choice for localized infections like CDI. Typical oral administration results in less than 10% bioavailability, meaning very little of the active drug enters the main circulation. The majority of the administered dose passes through the digestive system and is then excreted unchanged in the feces.
In contrast, IV vancomycin, used for systemic infections, is cleared predominantly by the kidneys, with more than 80% of the dose excreted unchanged in the urine. While absorption is minimal, in patients with severe intestinal inflammation, such as active pseudomembranous colitis, there can be a slight increase in systemic absorption. This small amount of absorption can occasionally lead to detectable serum concentrations, which requires caution in compromised patients.
Standard Dosing for Infection
The dosage of oral vancomycin is tailored to the severity of the C. difficile infection (CDI) being treated. For an initial episode of non-severe CDI, the standard adult regimen is 125 milligrams taken four times daily for 10 days. This dose is highly effective because it produces stool concentrations far exceeding the minimum needed to inhibit the bacteria.
In cases of fulminant CDI, which involves complications like shock or ileus (intestinal paralysis), a higher dose may be prescribed. The recommended dosage for these severe cases is 500 milligrams four times daily for the same 10-day duration. This higher dose ensures adequate drug levels are reached in the compromised colon.
For patients experiencing recurrent CDI, physicians may employ a tapered and pulsed regimen to prevent relapses. This involves starting at the standard dose and then gradually reducing the frequency over several weeks, rather than abruptly stopping the medication. Vancomycin is available as capsules or as a powder that is mixed into a liquid solution, offering flexibility for administration.
Potential Drug Interference
Because oral vancomycin is minimally absorbed into the systemic circulation, the risk of traditional drug-drug interactions is low. However, caution is necessary, particularly regarding drugs that affect the kidney or inner ear, which are known targets of vancomycin toxicity.
If a patient has severe underlying intestinal inflammation, the small amount of vancomycin that enters the bloodstream can pose a risk, especially when combined with other nephrotoxic agents. Medications like aminoglycoside antibiotics, loop diuretics, or certain intravenous contrast dyes can increase the combined risk of damage to the kidney (nephrotoxicity) or the inner ear (ototoxicity). Close monitoring is advised for patients receiving these combinations. Additionally, certain gut-acting medications, such as cholestyramine, can bind to vancomycin within the digestive tract, potentially reducing its local concentration and therapeutic effectiveness.