Cephalexin 500 mg Dosing for Adults
For adults with mild to moderate bacterial infections and normal renal function, the standard dose of cephalexin is 500 mg every 6 hours (four times daily), with a total daily dose ranging from 1 to 4 grams depending on infection severity. 1
Standard Dosing Regimens
The FDA-approved dosing for cephalexin is 250 mg every 6 hours for routine infections, with 500 mg every 6 hours recommended for more severe infections or less susceptible organisms. 1 The Infectious Diseases Society of America specifically recommends 500 mg four times daily for skin and soft tissue infections caused by methicillin-susceptible Staphylococcus aureus (MSSA). 2, 3
Alternative Dosing for Specific Infections
For uncomplicated cystitis, streptococcal pharyngitis, and uncomplicated skin infections: 500 mg every 12 hours (twice daily) is FDA-approved for patients over 15 years of age, with treatment duration of 7-14 days for cystitis. 1
For urinary tract infections: Recent evidence demonstrates that 500 mg twice daily is as effective as four-times-daily dosing, with no difference in treatment failure rates (12.7% vs 17%, P=0.343) and potentially improved adherence. 4
Infection-Specific Considerations
Skin and Soft Tissue Infections
Cephalexin 500 mg four times daily provides excellent coverage against MSSA and streptococcal species. 2, 3 However, critical pitfalls exist:
Do not use cephalexin if MRSA is suspected - it has zero activity against methicillin-resistant organisms. Switch to trimethoprim-sulfamethoxazole, doxycycline, or clindamycin instead. 3
Cephalexin has poor anaerobic coverage - avoid for deep abscesses or infections likely involving anaerobes. 2
Limited activity against Pasteurella multocida and Eikenella corrodens - not ideal for animal or human bite wounds without combination therapy. 2
Respiratory Tract Infections
Cephalexin should NOT be used for acute bacterial sinusitis or many respiratory infections due to poor coverage of Haemophilus influenzae and Moraxella catarrhalis. 2 It is completely ineffective against Pseudomonas aeruginosa. 2
Streptococcal Pharyngitis
For streptococcal pharyngitis, cephalexin must be administered for at least 10 days to prevent rheumatic fever, regardless of symptom resolution. 1, 2
When to Escalate Therapy
If daily doses exceeding 4 grams are required, switch to parenteral cephalosporins (e.g., cefazolin 1 g IV every 8 hours) rather than increasing oral cephalexin beyond FDA-approved limits. 1, 2
Dosing Schedule for Optimal Adherence
Four-times-daily dosing should be spaced every 6 hours (e.g., 6:00 AM, 12:00 PM, 6:00 PM, 12:00 AM) to maintain consistent antibiotic levels. 2 For infections where twice-daily dosing is appropriate, spacing doses 12 hours apart improves adherence without compromising efficacy. 4, 5
Renal Dose Adjustment
Patients with creatinine clearance less than 30 mL/min require dose reduction proportional to their reduced renal function, as 70-100% of cephalexin is renally excreted unchanged within 6-8 hours. 6
Allergy Considerations
Cephalexin can be used in patients with non-severe penicillin allergy (e.g., delayed rash), but is contraindicated in those with immediate hypersensitivity reactions such as anaphylaxis, angioedema, or urticaria to penicillins due to potential cross-reactivity. 3