Vancomycin is a glycopeptide antibiotic, a class of drugs derived from natural bacterial metabolites that work by blocking bacteria from building their cell walls. It targets only gram-positive bacteria and is best known as a critical treatment for MRSA (methicillin-resistant Staphylococcus aureus) and severe Clostridioides difficile infections.
How Glycopeptide Antibiotics Work
Glycopeptide antibiotics are structurally complex molecules built around a rigid peptide backbone with sugar molecules attached. Vancomycin specifically is a tricyclic heptapeptide, meaning it contains seven amino acids arranged in three interlocking ring structures. This rigid shape is what allows the drug to latch onto a very specific target on bacteria.
That target is the building block bacteria use to construct their cell walls. Bacteria assemble their outer wall from a mesh-like material called peptidoglycan. To link this mesh together, they rely on small peptide chains that end in a specific two-amino-acid sequence (D-alanyl-D-alanine). Vancomycin binds directly to that sequence, physically blocking the cross-linking step. Without a properly formed cell wall, the bacterium can’t maintain its structure and dies. This is fundamentally different from how penicillin-type antibiotics work. While penicillins disable the enzymes that perform the cross-linking, vancomycin blocks the raw material those enzymes need to work with.
What Vancomycin Treats
Vancomycin is active against gram-positive bacteria only. Gram-negative bacteria have an outer membrane that the large vancomycin molecule can’t penetrate. Within the gram-positive world, its most important targets include:
- MRSA and other staphylococci: Vancomycin has long been considered the go-to treatment when staph infections resist standard antibiotics. However, some medical centers have documented a gradual decline in vancomycin’s potency against staph, sometimes called “MIC creep.” Staph infections caused by strains with reduced susceptibility have been linked to higher mortality rates regardless of infection site.
- Streptococci: Including viridans group streptococci, which can cause serious bloodstream infections.
- Clostridioides difficile: A common cause of severe, antibiotic-associated diarrhea. This is the primary reason vancomycin is given by mouth.
- Other gram-positive organisms: Including Bacillus species, Corynebacterium, and certain other bacteria that occasionally cause infections in hospitalized or immunocompromised patients.
IV Versus Oral: Two Different Uses
Vancomycin is unusual in that it’s given two completely different ways for two completely different purposes, and the reason comes down to absorption. When swallowed, vancomycin is barely absorbed from the digestive tract into the bloodstream. For most antibiotics, that would be a problem. For vancomycin, it’s actually useful.
For C. difficile infections, which live in the colon, oral vancomycin delivers the drug directly where it’s needed without entering the rest of the body. The standard adult dose is 125 mg taken four times daily for 10 days.
For infections elsewhere in the body, such as bloodstream infections, bone infections, or pneumonia caused by MRSA, vancomycin must be given intravenously. Once in the bloodstream, it penetrates well into many body compartments including the fluid surrounding the lungs, heart, and joints, as well as bile.
Key Side Effects
The most recognizable side effect is sometimes called “red man syndrome,” a flushing reaction that causes redness and itching across the face, neck, and upper body. This isn’t a true allergy. It happens because vancomycin directly triggers immune cells called mast cells to release histamine, and it’s tied to how fast the drug is infused. Slowing the infusion rate to at least 100 minutes per gram largely prevents it.
Kidney damage is the more serious concern, particularly with prolonged use or higher doses. In one study tracking patients on long-term vancomycin, 29% had their treatment stopped because of acute kidney injury. The risk climbs when patients receive four grams or more per day. For this reason, blood levels of vancomycin are routinely monitored during IV treatment to balance effectiveness against kidney safety.
Hearing damage is possible but appears to be less common. In the same long-term study, about 8% of patients experienced some degree of hearing change, with half of those cases being mild. No clear risk factors predicted who would develop hearing problems. Changes typically appeared within the first two to four weeks of treatment.
Vancomycin Resistance
Some bacteria have evolved the ability to dodge vancomycin entirely. The most clinically significant are vancomycin-resistant enterococci, or VRE. These bacteria modify the target that vancomycin normally binds to, swapping out the D-alanyl-D-alanine ending for a slightly different structure that vancomycin can’t grip. The result is that the drug simply can’t attach and has no effect.
VRE is primarily a hospital-acquired problem. In 2017, VRE caused an estimated 54,500 infections among hospitalized patients in the United States and roughly 5,400 deaths, according to the CDC. VRE infections are especially dangerous because they tend to strike patients who are already critically ill, and treatment options are limited to a handful of newer antibiotics.
There is also growing concern about vancomycin-resistant strains of Staphylococcus aureus, though fully resistant staph remains rare. The more common issue is the gradual reduction in vancomycin sensitivity among staph strains. When staph isolates show reduced susceptibility, clinical guidelines recommend evaluating whether the patient is actually improving on vancomycin, and switching to an alternative if they’re not.