What is the recommended dosage of cephalexin (cephalosporin antibiotic) for adults?

Cephalexin Dosage for Adults

For most infections in adults, cephalexin should be dosed at 500 mg four times daily (every 6 hours), which is the standard recommendation from the Infectious Diseases Society of America and FDA labeling. 1, 2

Standard Adult Dosing Regimens

Most Common Infections

• 500 mg four times daily (every 6 hours) is the standard dose for skin and soft tissue infections, particularly those caused by methicillin-susceptible Staphylococcus aureus (MSSA) 3, 1, 2
• The total daily dose ranges from 1 to 4 grams divided throughout the day 2
• For mild to moderate infections, the dose can be reduced to 250 mg every 6 hours 1, 2

Simplified Dosing Options

• 500 mg twice daily (every 12 hours) is FDA-approved for specific infections: streptococcal pharyngitis, uncomplicated skin infections, and uncomplicated cystitis in patients over 15 years of age 2
• Recent evidence demonstrates that 500 mg twice daily is as effective as four-times-daily dosing for uncomplicated urinary tract infections, with potential benefits for adherence 4

Infection-Specific Dosing

Skin and Soft Tissue Infections

• 500 mg three to four times daily for 7-10 days 3, 1, 5
• This provides adequate coverage against S. aureus and Streptococcus species 5
• If no improvement occurs within 5 days, extend treatment duration 5

Urinary Tract Infections

• 500 mg every 12 hours for uncomplicated cystitis, continued for 7-14 days 2, 4
• Achieves urinary concentrations of 500-1000 mcg/mL, far exceeding the minimum inhibitory concentration for common uropathogens 6

Bite Wounds (Animal or Human)

• 500 mg three to four times daily when used as monotherapy 3
• Note: Cephalexin has limitations for bite wounds—it misses Pasteurella multocida (animal bites) and Eikenella corrodens (human bites), so combination therapy or alternative agents are often preferred 3

Critical Limitations and When NOT to Use Cephalexin

Ineffective Against:

• MRSA (methicillin-resistant S. aureus): Completely ineffective; use trimethoprim-sulfamethoxazole, clindamycin, or doxycycline instead 1, 5
• Respiratory pathogens: Poor activity against Haemophilus influenzae and Moraxella catarrhalis; inappropriate for sinusitis and many respiratory infections 1
• Pseudomonas aeruginosa: No activity whatsoever 1
• Anaerobes: Limited coverage, making it unsuitable for deep abscesses or anaerobic infections 3, 5

Allergy Considerations:

• Can be used in patients with non-severe penicillin allergy 1
• Contraindicated in patients with history of anaphylaxis, angioedema, or urticaria to penicillins due to cross-reactivity risk 1, 5

Dosing Adjustments

Renal Impairment:

• Patients with creatinine clearance <30 mL/min require dose reduction proportional to reduced renal function 6
• 70-100% of the dose is excreted unchanged in urine within 6-8 hours 6

Severe Infections:

• If daily doses exceeding 4 grams are required, switch to parenteral cephalosporins (e.g., cefazolin 1 g IV every 8 hours) 3, 2

Duration of Therapy

• 7-10 days for most skin and soft tissue infections 1, 5
• 7-14 days for uncomplicated cystitis 2
• At least 10 days for β-hemolytic streptococcal infections to prevent rheumatic fever 2

Common Pitfalls to Avoid

• Do not use cephalexin for suspected MRSA infections—it will fail 1, 5
• Do not use for acute bacterial sinusitis due to inadequate H. influenzae coverage 1
• Do not use for endocarditis prophylaxis—96% of viridans group streptococci are resistant 1
• For simple skin abrasions without systemic signs of infection, topical antimicrobials may be sufficient; avoid unnecessary systemic antibiotics 5