Drug Classification

Cephalexin is a broad-spectrum, first-generation cephalosporin antibiotic. It is bactericidal, meaning it kills bacteria rather than merely inhibiting their growth. Cephalexin disrupts the synthesis of the bacterial cell wall, leading to cell lysis and death.

Mechanism of Action

Cephalexin interferes with bacterial cell wall synthesis by binding to specific penicillin-binding proteins (PBPs) within the bacterial cell wall. This inhibition results in the formation of a defective cell wall and leads to cell lysis. Due to its action on PBPs, Cephalexin is effective primarily against gram-positive bacteria and some gram-negative bacteria.

Absorption and Distribution

Cephalexin is well-absorbed orally, with peak plasma concentrations achieved approximately one hour after ingestion. It is widely distributed throughout the body, including into the lungs, liver, bile, kidneys, and heart. The drug does not significantly penetrate into the cerebrospinal fluid.

Metabolism and Excretion

Cephalexin is minimally metabolized in the body and is primarily excreted unchanged in the urine. The elimination half-life of Cephalexin is approximately 0.5 to 1.2 hours, and it may be prolonged in patients with renal impairment. The drug is eliminated primarily by glomerular filtration and tubular secretion.

Therapeutic Indications

Cephalexin is indicated for the treatment of bacterial infections such as respiratory tract infections, otitis media, skin and skin structure infections, bone infections, and genitourinary tract infections. It is commonly prescribed for conditions like streptococcal pharyngitis, uncomplicated cystitis, and cellulitis.

Off-Label Uses

Off-label uses of Cephalexin include the treatment of mastitis, some sexually transmitted infections, and for bacterial endocarditis prophylaxis in patients undergoing certain dental or surgical procedures.

Dosage Forms

Cephalexin is available in multiple dosage forms: oral capsules, oral tablets, and oral suspension. The oral capsule form is most commonly available in strengths of 250 mg and 500 mg. The oral suspension is typically provided in concentrations of 125 mg/5 mL and 250 mg/5 mL.

Standard Dosage

For adults, the usual dosage of Cephalexin is 250 mg to 500 mg every 6 hours, depending on the severity of the infection. For pediatric patients, the common dose is 25-50 mg/kg/day in divided doses. In more severe infections, the dosage may be doubled.

Administration Guidelines

Cephalexin should be taken with a full glass of water. It can be taken with or without food, although taking with food may reduce gastrointestinal upset. The medication should be taken at evenly spaced intervals to maintain optimal blood levels.

Storage Conditions

Cephalexin capsules and tablets should be stored at room temperature, between 20°C and 25°C (68°F and 77°F). The oral suspension should be refrigerated and can be kept for up to 14 days. Any unused portion of the suspension should be discarded after this period.

Pharmacokinetic Interactions

Concurrent use of Cephalexin with probenecid can decrease renal excretion of the antibiotic, leading to elevated blood levels. Cephalexin may also enhance the effects of anticoagulants such as warfarin by impacting intestinal vitamin K production. Significant drug interactions should be reviewed prior to initiating therapy.

Specific Contraindications

Patients with known hypersensitivity to cephalosporins should not use Cephalexin. It is also contraindicated in patients with a history of severe allergic reactions to penicillins due to potential cross-reactivity.

Pregnancy and Lactation

Cephalexin is categorized as pregnancy category B. Animal studies have not shown harm to the fetus, but there are no well-controlled studies in pregnant women. Cephalexin is also excreted in breast milk; hence, caution should be exercised when administered to nursing mothers.

Adverse Reactions

Common adverse reactions to Cephalexin include gastrointestinal disturbances such as diarrhea, nausea, vomiting, and abdominal pain. Hypersensitivity reactions such as rash, urticaria, and pruritus are also reported. More severe reactions include anaphylaxis, Stevens-Johnson syndrome, and Clostridium difficile-associated diarrhea.

Laboratory Test Interferences

Cephalexin can affect some laboratory tests. It may result in false-positive results for glucose in urine tests using non-enzymatic methods. Also, it may interfere with the Coombs test, which is used to detect immune hemolytic anemia.

Patient Counseling Points

Patients should be instructed to complete the full course of therapy, even if they feel better, to prevent resistance development. They should report any signs of an allergic reaction immediately. It’s also important for patients to maintain hydration and inform their healthcare provider of all medications they are taking to avoid possible interactions.