What are the recommended dosing regimens for cephalexin in adults and children for common infections, including adjustments for renal impairment?

Cephalexin Dosing Guidelines

Adult Dosing

For most common infections in adults with normal renal function, prescribe cephalexin 500 mg orally every 6 hours (four times daily). 1, 2

Standard Adult Regimens

• Skin and soft tissue infections (MSSA): 500 mg every 6 hours for 7–10 days 1, 2
• Respiratory tract infections: 500 mg every 6 hours 1, 2
• Uncomplicated cystitis (patients >15 years): 500 mg every 12 hours for 7–14 days 2
• Streptococcal pharyngitis: 500 mg every 12 hours for 10 days 2

The adult dosage range is 1–4 grams daily in divided doses; if daily doses exceeding 4 grams are required, switch to parenteral cephalosporins. 1, 2

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Pediatric Dosing

The usual recommended daily dosage for pediatric patients is 25–50 mg/kg/day divided into 4 doses (every 6 hours). 2

Indication-Specific Pediatric Dosing

• Mild to moderate infections: 25–50 mg/kg/day divided every 6 hours 1, 2
• MSSA skin and soft tissue infections: 75–100 mg/kg/day divided into 3–4 doses 1
• Otitis media: 75–100 mg/kg/day in 4 divided doses 2
• Streptococcal pharyngitis (patients >1 year) and skin infections: Total daily dose may be divided every 12 hours 2

For β-hemolytic streptococcal infections, administer cephalexin for at least 10 days to prevent rheumatic fever. 2

In severe infections, the pediatric dosage may be doubled. 2

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Renal Impairment Adjustments

Cephalexin should be administered with caution in patients with markedly impaired renal function; safe dosage may be lower than usually recommended. 2

• Patients with creatinine clearance <30 mL/min require a reduction in cephalexin dosage proportional to reduced renal function. 3
• Monitor serum creatinine or creatinine clearance to guide dose adjustments. 3
• Careful clinical observation and laboratory studies are mandatory under conditions of impaired renal function. 2

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Critical Clinical Considerations

When Cephalexin Is NOT Appropriate

• Lyme disease: Cephalexin is inactive against Borrelia burgdorferi and ineffective clinically. 4
• MRSA infections: Cephalexin has no activity against methicillin-resistant Staphylococcus aureus; use trimethoprim-sulfamethoxazole, clindamycin, or doxycycline instead. 1
• Diabetic foot infections: These polymicrobial wounds require broader gram-negative and anaerobic coverage. 1
• Bite wounds (animal or human): Use amoxicillin-clavulanate to cover Pasteurella multocida, Eikenella corrodens, and anaerobes. 1
• Necrotizing fasciitis or deep abscesses: Cephalexin lacks adequate anaerobic coverage; use piperacillin-tazobactam or vancomycin plus a carbapenem. 1
• Haemophilus influenzae, Moraxella catarrhalis, Pseudomonas aeruginosa: Cephalexin has poor or no activity against these organisms. 1

Penicillin Allergy Considerations

• Cephalexin may be used in patients with non-severe penicillin allergy (e.g., delayed rash). 1
• Contraindicated in patients with a history of anaphylaxis, angioedema, or urticaria to penicillins due to cross-reactivity risk. 1

Common Pitfalls

• Do not use three-times-daily dosing: Cephalexin's short half-life requires every-6-hour dosing to maintain effective tissue concentrations. 1
• Do not extend surgical prophylaxis beyond 24 hours: Prolonged prophylaxis increases resistance and Clostridioides difficile risk without additional benefit. 1
• Incision and drainage is primary treatment for abscesses: Antibiotics are adjunctive; do not rely on cephalexin alone for undrained collections. 1

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Drug Interactions

• Metformin: Cephalexin increases metformin Cmax and AUC by 34% and 24%, respectively, and decreases renal clearance by 14%; monitor metformin closely and adjust dose as needed. 2
• Probenecid: Inhibits renal excretion of cephalexin, increasing serum levels. 2

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Pregnancy and Lactation

• Pregnancy Category B: Reproduction studies in mice and rats showed no harm to the fetus; use during pregnancy only if clearly needed. 2
• Lactation: Cephalexin is excreted in breast milk (maximum level 4 mcg/mL at 4 hours post-dose, disappears by 8 hours); exercise caution when administering to nursing women. 2

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Practical Dosing Examples

Adult with Skin Infection (70 kg)

• Cephalexin 500 mg orally every 6 hours for 7–10 days 1, 2

Child with MSSA Infection (20 kg)

• Cephalexin 75–100 mg/kg/day = 1500–2000 mg/day divided into 3–4 doses
• Using 250 mg/5 mL suspension: 7.5–10 mL every 6–8 hours 1, 2

Child with Streptococcal Pharyngitis (15 kg)

• Cephalexin 25–50 mg/kg/day = 375–750 mg/day divided every 12 hours
• Using 250 mg/5 mL suspension: 3.75–7.5 mL twice daily for 10 days 2