Overview
Cefalexin (also spelled cephalexin) is a foundational first-generation cephalosporin antibiotic. Since its introduction, it has remained a staple in outpatient medical practice due to its excellent oral bioavailability, proven efficacy, and generally benign side effect profile. Its spectrum of activity is primarily focused on Gram-positive bacteria, particularly Staphylococci and Streptococci, making it an excellent choice for uncomplicated skin and soft tissue infections (like cellulitis and impetigo). It also provides sufficient coverage against certain Gram-negative organisms (like E. coli) to be highly effective for uncomplicated urinary tract infections (UTIs) and some respiratory tract infections.

Mechanism of Action
Cefalexin functions as a bactericidal agent by disrupting the synthesis of the bacterial peptidoglycan cell wall. It binds to specific penicillin-binding proteins (PBPs) located on the inner membrane of the bacterial cell wall. This binding blocks the transpeptidation stage of cell wall synthesis, which is essential for cross-linking the peptidoglycan chains. The resulting structural weakness causes the bacterial cell to lyse and die due to osmotic pressure. It is susceptible to destruction by beta-lactamases produced by many Gram-negative bacteria, limiting its broader use.

Dosage and Administration
For adults, the usual oral dosage ranges from 250 mg every 6 hours to 500 mg every 12 hours, depending on the nature and severity of the infection. For more severe infections, up to 4 grams daily in divided doses may be administered. For uncomplicated UTIs or skin infections, 500 mg every 12 hours is a common and convenient regimen. Pediatric dosing is typically 25 to 50 mg/kg/day divided into two to four doses. Cefalexin is well absorbed from the gastrointestinal tract and can be taken with or without food.

Side Effects and Risks
Cefalexin is generally very well tolerated. The most common side effects are mild gastrointestinal disturbances, including nausea, vomiting, diarrhea, and abdominal discomfort. As with all antibiotics, there is a risk of developing Clostridioides difficile-associated diarrhea, though it is less common than with broader-spectrum agents. Allergic reactions, ranging from mild skin rashes (urticaria, maculopapular rash) to severe anaphylaxis, can occur. Genital candidiasis (yeast infections) may also occur due to the disruption of normal flora.

Contraindications
The absolute contraindication for cefalexin is a known hypersensitivity to cefalexin or any other cephalosporin antibiotic. It should be used with significant caution in patients with a documented history of severe, immediate hypersensitivity reactions (anaphylaxis) to penicillins, due to the risk of cross-reactivity. Dose reductions are necessary for patients with severe renal impairment (creatinine clearance less than 30 mL/min) to prevent drug accumulation and potential toxicity.

Pregnancy and Lactation
Cefalexin is widely regarded as safe for use during pregnancy and is often a first-line choice for treating UTIs in pregnant women. Animal reproduction studies and extensive human experience have not revealed any evidence of teratogenicity or harm to the fetus. Cefalexin is excreted in human breast milk in small amounts. It is generally considered safe and compatible with breastfeeding, though the infant should be monitored for potential minor side effects such as diarrhea or thrush.