Cephalexin Dosing for Adults
For adults, cephalexin is dosed at 250 mg every 6 hours (four times daily) for most infections, with 500 mg every 12 hours (twice daily) as an acceptable alternative for uncomplicated skin infections, streptococcal pharyngitis, and uncomplicated cystitis 1.
Standard Adult Dosing
The FDA-approved dosing range is 1-4 grams daily in divided doses 1:
- Mild to moderate infections: 250 mg every 6 hours (QID)
- Uncomplicated skin infections, streptococcal pharyngitis, and uncomplicated cystitis: 500 mg every 12 hours (BID) is acceptable 1
- More severe infections or less susceptible organisms: Higher doses may be needed, up to 4 grams daily
- Cystitis duration: Continue for 7-14 days 1
Recent evidence supports that 500 mg twice daily is as effective as four-times-daily dosing for uncomplicated urinary tract infections, with the advantage of improved adherence 2.
For skin and soft tissue infections specifically, the IDSA guidelines recommend 500 mg four times daily orally for methicillin-susceptible Staphylococcus aureus (MSSA) infections 3.
Renal Dose Adjustments
Cephalexin requires dose adjustment in renal impairment because it is primarily renally eliminated 4, 5:
- Normal renal function (CrCl >100 mL/min): Standard dosing (half-life ~40-58 minutes)
- Moderate renal impairment: Extend dosing interval based on creatinine clearance
- Severe renal impairment/anephric patients: Half-life increases dramatically to 2.3-15.4 hours 4
A dosage nomogram should be used to calculate appropriate loading doses based on patient weight and maintenance doses based on corrected creatinine clearance 4. For patients with significant renal tubular dysfunction (not just glomerular), consider using both creatinine clearance and PSP15' testing for more accurate dosing 5.
Penicillin Allergy Considerations
Cephalexin can be safely used in most patients with penicillin allergy, but requires careful consideration of the allergy history 6, 7:
When Cephalexin is Safe:
- Patients with unverified penicillin allergy labels: Less than 5% have true allergy; cephalexin can be used as first-line 7
- Patients with confirmed penicillin allergy (non-aminopenicillin): Cross-reactivity risk is extremely low 6, 7
- Patients with low-risk penicillin allergy history: Risk of severe cross-reactivity is extremely low 7
Higher Risk Scenario:
- Confirmed aminopenicillin (amoxicillin/ampicillin) allergy: Cephalexin shares similar R1 side chains with aminocephalosporins, resulting in 16.45% cross-reactivity rate on skin testing 6
- In this specific population, consider alternatives or proceed with caution
Contraindications:
Do NOT use cephalexin in patients with history of:
- Severe delayed immunologic reactions (SCARs)
- Hemolytic anemia from beta-lactams 8
- Drug-induced liver injury from beta-lactams
- Acute interstitial nephritis from beta-lactams 6
Alternative for True Penicillin Allergy:
If cephalexin is contraindicated, clindamycin 300-400 mg four times daily is the preferred alternative for MSSA skin infections 3.
Important Caveats
- Cefazolin (IV first-generation cephalosporin) has unique side chains and demonstrates very low cross-reactivity with penicillins (0.7-0.8% in confirmed penicillin allergy), making it safer than cephalexin for penicillin-allergic patients 6, 9
- Rare but serious adverse effect: Cephalexin can cause drug-induced hemolytic anemia leading to acute renal failure requiring dialysis 8. Monitor for hematuria, anuria, or acute hemoglobin drop.
- Second and third-generation cephalosporins show negligible cross-reactivity with penicillins (OR 0.5-1.1) compared to first-generation agents like cephalexin (OR 5.8) 10