Cephalexin is an oral antibiotic belonging to the first-generation cephalosporin class, widely used to treat bacterial infections. It is part of the larger family of beta-lactam antibiotics, sharing similarities with penicillin but possessing a different core chemical structure. Approved by the U.S. Food and Drug Administration (FDA) in 1970, Cephalexin has become one of the most commonly prescribed oral antibiotics worldwide due to its effectiveness and favorable safety profile. It is intended for treating infections caused by susceptible bacteria and is ineffective against viral illnesses, such as the common cold. Available in capsule, tablet, and liquid suspension forms, it is a versatile option for adult and pediatric patients.
How Cephalexin Stops Bacteria
Cephalexin functions as a bactericidal agent, meaning it actively destroys the target bacteria rather than just preventing their growth. The drug achieves this effect by interfering with the integrity of the bacterial cell wall, a rigid outer layer that provides structural support. This cell wall is built through the cross-linking of large molecules called peptidoglycans, a process completed by specialized bacterial enzymes.
The drug’s mechanism of action centers on its ability to mimic the natural substrates of these enzymes, known as penicillin-binding proteins (PBPs). Cephalexin possesses a characteristic four-membered beta-lactam ring that binds irreversibly to the active site of the PBPs. By blocking the PBP enzymes, Cephalexin prevents the final step of peptidoglycan synthesis, which is the transpeptidation reaction necessary for cross-linking.
The inhibition of this cross-linking process results in a severely weakened and defective cell wall. Without this structural integrity, the bacterial cell ruptures and dies, a process known as autolysis. Bacteria can develop resistance to this drug, primarily by producing beta-lactamase enzymes that hydrolyze the beta-lactam ring, or by modifying the PBPs to reduce the drug’s binding affinity.
What Infections Cephalexin Treats
Cephalexin’s spectrum of activity is primarily directed against Gram-positive bacteria, which have a thick peptidoglycan layer that the drug effectively targets. It is particularly effective against common organisms such as Staphylococcus aureus (excluding MRSA) and Streptococcus pyogenes. The medication also shows moderate activity against certain Gram-negative bacteria, including Escherichia coli and Proteus mirabilis.
The drug is commonly prescribed for a range of infections across different body systems. In skin and soft tissue, Cephalexin is a frequent choice for treating conditions like cellulitis, impetigo, and abscesses. Its efficacy against S. pyogenes also makes it a standard treatment for respiratory tract infections, such as streptococcal pharyngitis, commonly known as strep throat.
Cephalexin is FDA-approved for treating otitis media (middle ear infections), particularly those caused by S. pneumoniae. It is also widely used for genitourinary tract infections, including both acute and chronic urinary tract infections (UTIs). Its use in these areas is aided by its effective distribution into the urinary tract.
The Body’s Handling of Cephalexin
The journey of Cephalexin through the body, known as its pharmacokinetics, begins with oral administration, characterized by rapid and nearly complete absorption. The drug is acid-stable, allowing it to be taken with or without food, though food may slightly delay the absorption rate. Peak plasma concentrations are typically reached quickly, often within one hour of dosing in healthy adults.
Once absorbed, Cephalexin distributes widely throughout the body, reaching therapeutic concentrations in various fluids and tissues, including bone, skin, and the urinary tract. Only a small fraction of the drug, about 10% to 15%, binds to plasma proteins. This means that most of the drug remains free and active to fight the infection.
A characteristic of Cephalexin is its minimal metabolism; it undergoes virtually no detectable biotransformation. The drug primarily leaves the body in its original, active form. The elimination process occurs overwhelmingly through the kidneys, with approximately 90% of the unchanged drug excreted in the urine via both glomerular filtration and active tubular secretion.
This high concentration of active drug in the urine contributes to its effectiveness in treating UTIs. For healthy adults, the serum half-life is short, ranging from about 0.5 to 1.2 hours. However, in patients with reduced kidney function, this half-life can be significantly prolonged, necessitating careful dose adjustments to prevent drug accumulation.
Patient Safety and Practical Use
When taking Cephalexin, patients should be aware of potential adverse effects. The most common are gastrointestinal issues such as diarrhea, nausea, and vomiting. These effects are generally mild, but persistent or severe diarrhea may indicate a more serious condition, such as Clostridioides difficile infection. Less common side effects can include skin rashes, dizziness, or headache.
A consideration for patient safety is the potential for allergic reactions, which can range from mild skin eruptions to severe anaphylaxis. Patients with a known allergy to penicillin should inform their healthcare provider. Cephalexin belongs to the same class of beta-lactam antibiotics and carries a risk of cross-reactivity.
Although the historical cross-reactivity rate of 10% is now considered an overestimate for many cephalosporins, Cephalexin has a side chain structure similar to some penicillins (like amoxicillin), which may increase the risk for cross-reaction in sensitive individuals.
For optimal treatment, it is important to complete the entire course of Cephalexin as prescribed, even if symptoms improve quickly. Stopping the medication prematurely risks incomplete treatment of the infection and can contribute to the development of antibiotic-resistant bacteria. Cephalexin interacts notably with probenecid, a medication used to treat gout, which interferes with the kidney’s ability to excrete the antibiotic, leading to higher blood levels.