There are roughly 15 to 20 major classes of antibiotics used in medicine today, depending on how finely you draw the lines between them. The exact count varies because some classification systems split closely related drugs into separate groups while others lump them together. The WHO’s list of medically important antimicrobials, for instance, identifies more than 20 distinct categories authorized for human use alone. What matters more than the precise number is understanding what these classes do differently and why doctors choose one over another.
The Major Antibiotic Classes
Antibiotics are grouped into classes based on their chemical structure and how they attack bacteria. The classes most commonly prescribed include penicillins, cephalosporins, macrolides, fluoroquinolones, tetracyclines, aminoglycosides, carbapenems, and sulfonamides. Beyond these well-known groups, there are more specialized classes: glycopeptides and lipoglycopeptides (vancomycin being the most familiar), monobactams, oxazolidinones, lincosamides (like clindamycin), polymyxins, streptogramins, ansamycins, lipopeptides, and nitroimidazoles, among others.
The CDC’s outpatient prescribing data shows that the bulk of antibiotics Americans actually receive fall into a smaller set: penicillins, cephalosporins, macrolides, fluoroquinolones, and tetracyclines dominate retail pharmacy prescriptions. The more specialized classes tend to be reserved for serious or drug-resistant infections treated in hospitals.
How Antibiotics Kill or Stop Bacteria
Despite the long list of classes, antibiotics really only use a handful of strategies against bacteria. Some destroy the bacterial cell wall, which causes the bacterium to burst. Penicillins, cephalosporins, carbapenems, monobactams, and glycopeptides all work this way, though each targets slightly different steps in wall construction. This is why they’re often grouped together under the umbrella term “beta-lactams” (except glycopeptides, which use a different mechanism to hit the same target).
Other antibiotics interfere with protein production inside the bacterium, essentially starving it of the molecular machinery it needs to grow and reproduce. Macrolides, tetracyclines, aminoglycosides, oxazolidinones, and lincosamides all fall into this camp. A third group disrupts the bacterium’s ability to copy or read its own DNA. Fluoroquinolones are the most widely prescribed example. Finally, sulfonamides and trimethoprim block folic acid production, a metabolic pathway bacteria need but humans get from food.
These different attack strategies explain why doctors sometimes combine antibiotics from two classes. Pairing a cell wall-targeting drug with an aminoglycoside, for example, can hit bacteria from two angles at once.
Broad-Spectrum vs. Narrow-Spectrum
You’ll often hear antibiotics described as “broad-spectrum” or “narrow-spectrum,” but these terms are less precise than they sound. The distinction dates back to the 1950s, when chloramphenicol and the first tetracyclines could kill a wide range of bacteria while penicillin G only worked against a narrow set. No formal definition of “broad” or “narrow” has ever been established, and in practice it’s more of a sliding scale. Tetracyclines are broader than some cephalosporins, which are broader than macrolides, which are broader than metronidazole.
What this means for you: a broad-spectrum antibiotic is more likely to work when doctors aren’t sure exactly which bacterium is causing an infection, but it also kills more of your normal, helpful bacteria in the process. That’s one reason doctors prefer to use narrower drugs when they can identify the specific bug responsible.
How Your Body Receives Them
Most outpatient antibiotics come as pills or liquid you swallow. Oral forms exist for penicillins, cephalosporins, macrolides, fluoroquinolones, tetracyclines, sulfonamides, and several other classes. For more serious infections, antibiotics are given intravenously in a hospital setting, which delivers the drug directly into the bloodstream at higher concentrations. Some antibiotics, like certain aminoglycosides and carbapenems, are only available as IV formulations because they aren’t absorbed well through the gut.
Topical antibiotics applied to the skin or eyes make up a third category. These are useful for surface-level infections where you want high local drug concentration without exposing the rest of the body. Mupirocin, a pseudomonic acid, is a common example used for skin infections including those caused by staph bacteria.
Common Side Effects Across Classes
All antibiotics can cause digestive side effects because they don’t distinguish perfectly between harmful bacteria and the beneficial ones living in your gut. Nausea, diarrhea, and yeast infections are the most frequently reported problems across virtually every class. Rash and dizziness also show up regularly.
Some classes carry distinctive risks. Fluoroquinolones have been linked to tendon damage and nerve problems. Aminoglycosides can affect hearing and kidney function, which is why blood levels are monitored during treatment. The most serious digestive side effect across all classes is infection with C. difficile, a bacterium that can flourish when antibiotics wipe out competing gut bacteria. C. difficile causes severe diarrhea and, in rare cases, life-threatening colon damage.
How Bacteria Resist Antibiotics
Bacteria have evolved several defense strategies that can render entire antibiotic classes ineffective. Some bacteria produce enzymes that physically break down the drug before it can work. A well-known example: certain strains of Klebsiella pneumoniae produce carbapenemases, enzymes that destroy carbapenems and most other beta-lactam antibiotics. This is particularly alarming because carbapenems are often the last resort for serious infections.
Other bacteria install molecular pumps in their cell walls that actively push antibiotics back out before the drug reaches its target. Pseudomonas aeruginosa, a common cause of hospital-acquired infections, can pump out fluoroquinolones, beta-lactams, and several other drug classes simultaneously. A third strategy involves changing the shape of the target the antibiotic is designed to attack, so the drug no longer fits. Each of these mechanisms can spread between bacteria through shared DNA, which is how resistance jumps between species.
New Antibiotics Still Emerging
The pipeline for new antibiotics is thin compared to other drug categories, but approvals continue. In 2025 alone, the FDA approved two novel antibiotics: zoliflodacin for treating gonorrhea and gepotidacin for uncomplicated urinary tract infections. Both target bacterial enzymes involved in DNA replication, and they represent genuinely new chemical approaches rather than tweaks to existing classes.
New approvals matter because resistance is steadily eroding the effectiveness of older drugs. The WHO now tracks more than 20 classes of antimicrobials considered medically important, and for many of them, resistant bacteria already exist. Having diverse antibiotic classes available gives doctors options when first-line treatments fail, which is why the number of classes matters beyond simple categorization.