Cefalexin
Oral first-generation cephalosporin antibiotic for bacterial infections.
Cefalexin (sometimes spelled cephalexin) is an oral antibiotic used against various bacterial infections. It belongs to the first-generation cephalosporin class of β-lactam antibiotics and works by interfering with bacterial cell wall construction, which kills gram-positive bacteria and some gram-negative ones. Its mechanism is similar to that of intravenous cefazolin, but cefalexin can be taken by mouth.
This drug is effective for infections of the middle ear, bones and joints, skin, and urinary tract. It can also treat certain cases of pneumonia and strep throat, and is used to prevent bacterial endocarditis. However, it does not work against methicillin-resistant *Staphylococcus aureus* (MRSA), most *Enterococcus* species, or *Pseudomonas*. Like all antibiotics, it has no effect on viral infections such as the flu, common cold, or acute bronchitis. People with mild or moderate penicillin allergies may use cefalexin, but it is not advised for those with severe penicillin allergies.
Common side effects include stomach upset and diarrhea. Allergic reactions or infections with *Clostridioides difficile* (which can cause diarrhea) are also possible. Use during pregnancy or breastfeeding appears safe for the fetus. The drug can be given to children and adults over 65, though those with kidney problems may need a lower dose.
Cefalexin was developed in 1967 and first marketed in 1969 under the brand name Keflex. It is now available as a generic medication and is listed on the World Health Organization's List of Essential Medicines. In 2023, it was the 86th most prescribed drug in the United States, with over 7 million prescriptions. In Canada, it was the fifth most common antibiotic in 2013, and in Australia, it ranked among the top 10 most prescribed medications from 2017 to 2023.
**Medical uses** Cefalexin treats bacterial infections such as otitis media, streptococcal pharyngitis, bone and joint infections, pneumonia, cellulitis, and urinary tract infections. It can also prevent bacterial endocarditis and recurrent urinary tract infections. It is not effective against MRSA. For patients intolerant to penicillin, cefalexin is a useful alternative—for example, penicillin is preferred for streptococcal respiratory infections, but cefalexin can be substituted. Caution is needed because cross-sensitivity between cephalosporins and penicillins occurs in up to 10% of people with a documented penicillin allergy.
**Pregnancy and breastfeeding** In Australia, cefalexin is classified as category A, meaning no evidence of harm has been found after use in many pregnant women. It is generally safe during breastfeeding.
**Adverse effects** The most common side effects are gastrointestinal disturbances (nausea, vomiting, and especially diarrhea) and hypersensitivity reactions (skin rashes, hives, fever, and anaphylaxis). Pseudomembranous colitis and *C. difficile* infection have been reported. Less common but serious effects include bruising of the skin and yellowing of the skin or eyes. Signs of an allergic reaction include rash, itching, swelling, trouble breathing, or red, blistered, swollen, or peeling skin. Overall, cefalexin allergy occurs in less than 0.1% of patients, but in 1% to 10% of those with a penicillin allergy.
**Interactions** Probenecid delays renal excretion of cefalexin, similar to other β-lactam antibiotics. It should not be taken with dofetilide, live cholera vaccine, warfarin, or cholestyramine. Alcohol reduces the rate of absorption. Cefalexin can raise metformin levels in the body. Histamine H2 receptor antagonists (e.g., cimetidine, ranitidine) may reduce its effectiveness by delaying absorption and altering its antimicrobial action. Zinc supplements can also lower cefalexin levels.
**Pharmacology** *Mechanism of action*: Cefalexin is a bactericidal β-lactam antibiotic that inhibits synthesis of the peptidoglycan layer of bacterial cell walls. It closely resembles d-alanyl-d-alanine and binds irreversibly to the active site of penicillin-binding proteins (PBPs), which are essential for cell wall construction. It is most active against gram-positive cocci and moderately active against some gram-negative bacilli. However, bacteria that produce β-lactamase can hydrolyze the β-lactam ring, rendering the drug inactive and contributing to resistance.
*Pharmacokinetics*: Cefalexin is rapidly and almost completely absorbed from the gastrointestinal tract when taken orally. Absorption is slightly reduced if taken with food, but the medication can be taken without regard to meals. Peak blood levels occur about one hour after dosing, and levels increase roughly linearly over a dose range of 250 to 1,000 mg. Like most cephalosporins, it is not metabolized in the body. The elimination half-life is about 30 to 60 minutes in people with normal kidney function.
- field
- Antibiotic
- class
- First-generation cephalosporin
- first marketed
- 1969
- brand name
- Keflex
- route
- Oral
Lore & Background
It is available as a generic medication and is on the World Health Organization's List of Essential Medicines. Cefalexin can treat certain bacterial infections, including those of the middle ear, bone and joint, skin, and urinary tract. It may also be used for certain types of pneumonia and strep throat and to prevent bacterial endocarditis. Cefalexin is not effective against infections caused by methicillin-resistant Staphylococcus aureus (MRSA), most Enterococcus, or Pseudomonas. Like other antibiotics, cefalexin cannot treat viral infections, such as the flu, common cold or acute bronchitis. Common side effects include stomach upset and diarrhea. Allergic reactions or infections with Clostridioides difficile, a cause of diarrhea, are also possible. Use during pregnancy or breastfeeding does not appear to be harmful to the fetus. It can be used in children and those over 65 years of age. Those with kidney problems may require a decrease in dose.
Reader's Guide
Cefalexin is a widely used oral antibiotic belonging to the first-generation cephalosporin class. Its significance lies in its effectiveness against a range of common bacterial infections, including those of the middle ear, bone and joint, skin, and urinary tract, as well as certain types of pneumonia and strep throat. It also serves as a preventive agent for bacterial endocarditis. As a β-lactam antibiotic, it works by disrupting bacterial cell wall synthesis. A key advantage is its oral administration, which allows for outpatient treatment, unlike some other first-generation cephalosporins such as intravenous cefazolin. Cefalexin is a useful alternative for patients with mild or moderate penicillin allergies, though caution is advised due to possible cross-sensitivity. Its legacy is reflected in its inclusion on the World Health Organization's List of Essential Medicines and its consistent high prescription rates in multiple countries. However, its limitations are notable: it is ineffective against MRSA, most Enterococcus, Pseudomonas, and all viral infections. The development of bacterial resistance, particularly via β-lactamase enzymes, remains a concern. Overall, cefalexin remains a foundational antibiotic in primary care and hospital settings.
Did You Know?
- It is on the World Health Organization's List of Essential Medicines.
- Cefalexin is not effective against infections caused by methicillin-resistant Staphylococcus aureus (MRSA).
- More than 90% of cefalexin is excreted unchanged in the urine within 8 hours.
Molecular Mechanism & Pharmacokinetic Profile
Cefalexin belongs to the first-generation cephalosporin class of β-lactam antibiotics, sharing a close pharmacological relationship with intravenous cefazolin while offering the practical advantage of oral administration. Its bactericidal action centers on the bacterial cell wall: the molecule structurally mimics d-alanyl-d-alanine, the terminal amino acid sequence of the peptidoglycan layer, allowing it to lock irreversibly into the active site of penicillin-binding proteins that are indispensable for wall synthesis. This makes it particularly potent against gram-positive cocci, with only moderate reach into certain gram-negative bacilli. A critical vulnerability is the β-lactamase enzyme produced by some bacteria, which cleaves the β-lactam ring and neutralizes the drug, a key driver of emerging resistance. Pharmacokinetically, cefalexin is absorbed rapidly and nearly completely from the gut, reaching peak plasma concentrations roughly one hour after a dose. The drug is neither metabolized nor inactivated by the body; instead, over ninety percent is eliminated unchanged in urine within eight hours, with a half-life of thirty to sixty minutes in individuals with normal kidney function. Probenecid, by competing for renal tubular secretion, can slow this clearance.
Clinical Applications & Therapeutic Boundaries
Cefalexin occupies a broad but clearly delineated therapeutic niche in modern medicine. Clinicians prescribe it for middle-ear infections, streptococcal throat infections, bone and joint infections, cellulitis, certain pneumonias, and urinary-tract infections, and it also serves as prophylaxis against bacterial endocarditis and recurrent UTIs. A particularly important role is as a substitute for penicillin in patients who cannot tolerate that drug, though cross-sensitivity among β-lactam antibiotics has been documented in up to ten percent of individuals with a confirmed penicillin allergy, so caution remains essential. The drug is explicitly not recommended for those with severe penicillin hypersensitivity. Its spectrum has firm limits: it is ineffective against methicillin-resistant Staphylococcus aureus, the majority of Enterococcus species, and Pseudomonas, and like all antibiotics it has no activity against viral pathogens responsible for influenza, the common cold, or acute bronchitis. A web of drug interactions further shapes its use—probenecid, dofetilide, warfarin, cholestyramine, live cholera vaccine, alcohol, metformin, H2-receptor antagonists, and zinc supplements all alter its absorption, clearance, or efficacy in clinically meaningful ways.
Safety Profile & Considerations in Special Populations
The most frequently reported adverse effects of cefalexin mirror those of other oral cephalosporins: gastrointestinal upset, with diarrhea being the single most common complaint, alongside nausea and vomiting. Hypersensitivity manifestations range from skin rashes and urticaria to fever and, in rare instances, anaphylaxis. More serious but uncommon reactions include pseudomembranous colitis linked to Clostridioides difficile overgrowth, unexplained bruising, and jaundice. True cefalexin allergy is rare, affecting fewer than one in a thousand patients, though the incidence rises to one to ten percent among those with a known penicillin allergy. Warning signs of a severe reaction include widespread rash, intense itching, facial swelling, difficulty breathing, or skin that turns red, blisters, swells, or peels. For pregnant and breastfeeding women, the safety record is reassuring: Australian regulatory classification places it in Category A, reflecting the absence of observed fetal harm across many pregnancies, and lactation use is generally considered safe. The drug is appropriate for children and adults over sixty-five, although patients with impaired renal function typically require a reduced dose to avoid accumulation.
Development, Global Reach & Prescription Legacy
Today it is widely available as a generic medication, and the World Health Organization both lists it on its List of Essential Medicines and classifies it as highly important for human medicine—a dual endorsement that underscores its irreplaceable role in global health systems. Its prescription footprint is substantial and geographically diverse. The drug's naming also reflects its international journey: cefalexin serves as both the International Nonproprietary Name and the Australian Approved Name, while cephalexin is the British variant, a small linguistic reminder of how a single molecule travels across regulatory and cultural borders to reach patients worldwide.
Frequently Asked Questions
Who is Cefalexin?
Cefalexin (also spelled cephalexin) is a first-generation cephalosporin antibiotic that has been in clinical use since 1969, best known by its brand name Keflex. It is an oral β-lactam drug prescribed for a range of bacterial infections.
What are Cefalexin's powers/role?
Cefalexin eliminates gram-positive and select gram-negative bacteria by disrupting the synthesis of their cell walls. It mirrors the mechanism of its intravenous counterpart cefazolin, but its key advantage is that it can simply be swallowed as a tablet.
How does Cefalexin's story end?
Once the bacterial infection is cleared, Cefalexin's role is complete and the drug is cleared from the body, predominantly via the kidneys. A typical treatment course is short, and the 'arc' wraps up when the pathogen is gone.
Why is Cefalexin important?
As one of the earliest widely adopted oral cephalosporins, Cefalexin let doctors treat common bacterial infections outside the hospital without needing IV access. Its 1969 market launch made first-generation cephalosporin therapy a practical outpatient option.
More in Antibiotic Drugs 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record