Cefalexin

Cefalexin (also known as cephalexin) is an oral antibiotic used to treat various bacterial infections. It belongs to the first-generation cephalosporin class, a type of β-lactam antibiotic, and works by interfering with the construction of the bacterial cell wall, 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 antibiotic is effective against infections of the middle ear, bones and joints, skin, and urinary tract.

Medical uses

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, or Pseudomonas. Like all antibiotics, it is ineffective against viral infections such as the flu, common cold, or acute bronchitis. People with mild or moderate penicillin allergies can often take cefalexin, but it is not advised for those with severe penicillin allergies.

Adverse effects

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 does not appear to harm the fetus. It is safe for 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 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 medication 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.

Cefalexin treats bacterial infections such as otitis media (middle ear infection), streptococcal pharyngitis (strep throat), bone and joint infections, pneumonia, cellulitis (skin infection), and urinary tract infections. It may also be used to prevent bacterial endocarditis or recurrent urinary tract infections. It does not treat MRSA infections. For patients intolerant to penicillin, cefalexin is a useful alternative—for example, penicillin is the first choice for streptococcal respiratory infections, but cefalexin can be substituted.

Pregnancy and breastfeeding

However, caution is needed because cross-sensitivity between cephalosporins and penicillins occurs in up to 10% of people with a documented penicillin allergy. In Australia, cefalexin is classified as Category A, meaning no evidence of harm has been found after use by many pregnant women. It is generally safe during breastfeeding.

The most common side effects are gastrointestinal (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.

Interactions

Overall, cefalexin allergy occurs in less than 0.1% of patients, but in those with a penicillin allergy, the rate rises to 1% to 10%. Like other β-lactam antibiotics, probenecid delays renal excretion of cefalexin. It is not recommended to take cefalexin 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 efficacy by delaying absorption. Zinc supplements can also lower cefalexin levels in the body. Pharmacology Mechanism of action: Cefalexin is a bactericidal β-lactam antibiotic that inhibits synthesis of the peptidoglycan layer of the bacterial cell wall.

It resembles d-alanyl-d-alanine and binds irreversibly to 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, making 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 when taken with food, but it can be taken with or without meals. Peak levels occur about one hour after administration, and maximum 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.