Cephalosporins are a large family of antibiotics that kill bacteria by destroying their cell walls. They belong to the same broader class as penicillin (called beta-lactams) and are among the most widely prescribed antibiotics in the world. Doctors use them to treat everything from simple skin infections and strep throat to serious conditions like meningitis and sepsis. They’re organized into five generations, each designed to target a different range of bacteria.
How Cephalosporins Kill Bacteria
Bacteria survive by building and maintaining a rigid cell wall made of a mesh-like material called peptidoglycan. Cephalosporins interfere with the proteins bacteria use to assemble that wall. Without a functioning wall, the bacterial cell swells with water and bursts. This makes cephalosporins bactericidal, meaning they actively kill bacteria rather than just slowing their growth.
Because human cells don’t have cell walls at all, cephalosporins can target bacteria without directly damaging your own tissue. That selective action is a key reason they’re considered relatively safe compared to some other antibiotic classes.
The Five Generations
Cephalosporins are grouped into generations based on which bacteria they work against. The pattern is straightforward: earlier generations are strongest against common skin and respiratory bacteria (gram-positive), while later generations gain increasing power against harder-to-treat gram-negative bacteria, which have an extra protective outer membrane.
First Generation
These are the workhorses for everyday infections. Cephalexin (often sold as Keflex) is one of the most commonly prescribed antibiotics in primary care. First-generation cephalosporins cover most gram-positive cocci, the bacteria behind skin infections, strep throat, and simple urinary tract infections. They also work against a few gram-negative species like E. coli, Proteus mirabilis, and Klebsiella pneumoniae. They’re frequently given before surgery to prevent wound infections.
Second Generation
Second-generation drugs like cefuroxime expand coverage to include bacteria commonly responsible for sinus infections, ear infections, and bronchitis, particularly Haemophilus influenzae and Moraxella catarrhalis. Some second-generation cephalosporins also cover certain anaerobic bacteria (species that thrive without oxygen), making them useful for some abdominal infections.
Third Generation
This is where the shift toward gram-negative power becomes noticeable. Third-generation cephalosporins like ceftriaxone trade some gram-positive strength for much broader gram-negative coverage. They can cross the blood-brain barrier, which makes them a go-to choice for bacterial meningitis. They’re also a standard treatment for gonorrhea, pneumonia, complicated urinary tract infections, and bloodstream infections. Ceftriaxone is one of the most commonly used antibiotics in hospital emergency departments.
Fourth Generation
Fourth-generation cephalosporins like cefepime retain the gram-negative reach of the third generation but add activity against some bacteria that have developed resistance through beta-lactamase enzymes. These are typically reserved for serious hospital-acquired infections where resistant organisms are more likely.
Fifth Generation
The fifth generation was developed specifically to tackle drug-resistant bacteria that older cephalosporins cannot touch. Ceftaroline (brand name Teflaro) is active against MRSA (methicillin-resistant Staphylococcus aureus), a notoriously difficult-to-treat infection. In lab testing, ceftaroline proved at least eight times more potent than cefepime and at least 16 times more active than ceftriaxone against MRSA strains. It also works against strains with reduced susceptibility to vancomycin, one of the traditional last-resort drugs for MRSA.
Common Side Effects
Most people tolerate cephalosporins well. The most frequent side effects are digestive: nausea, diarrhea, and stomach cramps. Skin rashes and mild allergic reactions can also occur. These side effects are typically mild and resolve after you finish the course.
A more serious concern, particularly with third- and fourth-generation cephalosporins, is the risk of C. diff infection. The CDC lists these broader-spectrum cephalosporins as a notable risk factor for C. diff, which causes watery diarrhea, fever, abdominal pain, and loss of appetite. C. diff develops because powerful antibiotics can wipe out the normal protective bacteria in your gut, allowing C. diff to take over. If you develop persistent or worsening diarrhea during or after a cephalosporin course, that’s worth reporting promptly.
Penicillin Allergy and Cross-Reactivity
Because cephalosporins and penicillin share a similar chemical core, people with penicillin allergies sometimes worry about taking cephalosporins. The actual risk depends on the generation. First- and second-generation cephalosporins have a cross-reactivity rate of roughly 1% to 8% in people with confirmed penicillin allergy. Third-generation cephalosporins like ceftriaxone carry less than 1% cross-reactivity. For most people with a mild penicillin allergy (like a rash), cephalosporins are generally considered safe, though your prescriber will factor in the severity of your past reaction.
Kidney Function and Dosing
Most cephalosporins are cleared from the body through the kidneys. If your kidneys aren’t working at full capacity, the drug can build up to higher-than-intended levels. For this reason, doses are often adjusted based on your kidney function, measured through a blood test that estimates how efficiently your kidneys filter waste. Your provider may lower the dose, space it out further, or both.
How Bacteria Resist Cephalosporins
The biggest threat to cephalosporin effectiveness comes from bacterial enzymes called extended-spectrum beta-lactamases (ESBLs). These enzymes physically break apart the cephalosporin molecule before it can attack the cell wall. ESBL-producing bacteria can resist first-, second-, and third-generation cephalosporins, and some strains can also inactivate fourth-generation drugs. The genes coding for these enzymes spread between bacteria on mobile pieces of DNA called plasmids, which accelerates resistance.
About 90% of E. coli strains carrying one common resistance gene (TEM-1) can resist first-generation cephalosporins and ampicillin. More advanced variants of these enzymes can knock out third-generation drugs as well. This is a major reason doctors try to use the narrowest-spectrum cephalosporin that will work for a given infection, reserving the broader-spectrum options for situations that truly need them.
One creative solution is cefiderocol, a newer cephalosporin designed to bypass resistance entirely. It’s attached to a siderophore, a molecule that bacteria mistake for iron and actively pull inside their own cells. This “Trojan horse” approach lets cefiderocol slip past the outer defenses that block other antibiotics, making it effective against some of the most resistant gram-negative bacteria in existence, including strains that resist even carbapenems, which are typically considered last-resort drugs.