Cephalexin Dosing and Duration for Adults
For adults, the standard dose of cephalexin is 250 mg every 6 hours (1 g daily) for most infections, with 500 mg every 12 hours as an alternative for streptococcal pharyngitis, skin and soft tissue infections, and uncomplicated cystitis. 1 For severe infections or less susceptible organisms, doses up to 4 g daily in divided doses may be required. 1
Standard Dosing Regimens
Mild to Moderate Infections
- 250 mg every 6 hours (four times daily) is the usual adult dose 1
- 500 mg every 12 hours (twice daily) is acceptable for:
- Streptococcal pharyngitis
- Skin and soft tissue infections
- Uncomplicated cystitis in patients >15 years 1
Recent evidence supports twice-daily dosing as equally effective as four-times-daily regimens. Studies demonstrate no difference in treatment failure rates between cephalexin 500 mg twice daily versus four times daily for urinary tract infections (12.7% vs 17%, P=0.343) 2 and similar outcomes for UTIs diagnosed in emergency departments (18.7% vs 15.0%, P=0.465) 3. The twice-daily regimen offers improved adherence without compromising efficacy.
Severe Infections
- 500 mg every 6 hours or up to 1 g every 6 hours (maximum 4 g daily) 1
- If doses exceeding 4 g daily are needed, parenteral cephalosporins should be considered 1
Duration of Therapy
Treatment duration varies by infection type:
- Uncomplicated cystitis: 7-14 days 1
- Skin and soft tissue infections: 5-10 days, with most guidelines recommending 5 days minimum 4
- Streptococcal pharyngitis: At least 10 days for β-hemolytic streptococcal infections 1
- Respiratory tract infections: 10 days 5
- Complicated infections: Extended courses may be warranted; for example, MSSA bacteremic vertebral osteomyelitis showed success with median 29 days of oral cephalexin following initial IV therapy 6
Clinical Context from Guidelines
While cephalexin is not prominently featured in recent IDSA guidelines for many infections, it remains a guideline-recommended option for specific scenarios. For methicillin-susceptible Staphylococcus aureus (MSSA) skin infections, cephalexin 500 mg four times daily is listed as a preferred oral agent 7, 8. For pediatric pneumonia caused by MSSA, oral cephalexin at 75-100 mg/kg/day in 3-4 divided doses is preferred 9, 10.
The 2014 IDSA skin and soft tissue infection guidelines recommend β-lactams like cephalexin (500 mg four times daily) for non-purulent cellulitis targeting streptococci, with empiric MRSA coverage added only for specific risk factors 4.
Important Caveats
- Renal adjustment: Dosing must be adjusted for renal impairment, though specific parameters are not detailed in the FDA label 1
- Obesity considerations: Recent guidelines suggest standard β-lactam dosing is generally adequate for mild-moderate infections in obesity, with extended infusions and therapeutic drug monitoring reserved for critically ill patients 11
- Cross-reactivity: Cephalexin can be used in most penicillin-allergic patients, as true cross-reactivity is <5%, though slightly higher risk exists with amino-penicillin allergies 12
- Resistance patterns: Local susceptibility data should guide therapy, as resistance varies geographically 13
The twice-daily regimen (500 mg every 12 hours) represents a practical, evidence-based approach for most outpatient infections, improving adherence while maintaining efficacy comparable to traditional four-times-daily dosing. 2, 14, 3