Cefazolin Dosing for Cellulitis
For adults with cellulitis, use cefazolin 1 gram IV every 8 hours for standard dosing, with treatment duration of 5 days (extendable if not improved). 1
Adult Dosing Regimens
Standard Dosing (Non-Obese, Normal Renal Function)
- 1 gram IV every 8 hours is the recommended dose per IDSA guidelines 1
- Treatment duration: 5 days minimum, extend if infection has not improved 1
- Can be administered as once-daily regimen: 2 grams IV cefazolin plus 1 gram oral probenecid daily for moderate-to-severe cellulitis in outpatient/home settings 2, 3
Obesity Adjustments (≥120 kg)
Use 3 grams IV every 8 hours for patients weighing ≥120 kg. Recent evidence demonstrates that high-dose cefazolin (3 g/dose, up to 9 g/day) in patients ≥120 kg resulted in 63.7% probability of better outcomes compared to standard dosing, with no increase in adverse effects 4. This is supported by additional safety data showing high-dose cefazolin (9 g/day) is well-tolerated in high body weight patients 5.
The 2025 obesity pharmacokinetics guidelines note that higher than standard dosing is not routinely recommended for mild-moderate infections in obesity, but should be considered for severe infections 6. However, the specific cellulitis data in patients ≥120 kg supports the higher dosing strategy.
Renal Impairment Adjustments
Always give a loading dose appropriate to infection severity, then adjust maintenance dosing 7:
- CrCl ≥55 mL/min: Full dose (1 g every 8 hours)
- CrCl 35-54 mL/min: Full dose but extend interval to every 8 hours minimum (or every 12 hours)
- CrCl 11-34 mL/min: Half the usual dose every 12 hours
- CrCl ≤10 mL/min: Half the usual dose every 18-24 hours
Critical Warning: Failure to adjust for renal dysfunction can cause seizures and encephalopathy, particularly in elderly patients with pre-existing CNS conditions 8. A case report documented fatal tonic-clonic seizures in an 87-year-old receiving 2 g every 4 hours (should have been every 12 hours based on renal function).
Pediatric Dosing
Standard Pediatric Dosing
- Mild-to-moderate infections: 25-50 mg/kg/day divided into 3-4 equal doses 7
- Severe infections: Up to 100 mg/kg/day (maximum 45 mg/lb/day) 7
- Not recommended for premature infants and neonates 7
Pediatric Renal Impairment
After loading dose 7:
- CrCl 70-40 mL/min: 60% of normal daily dose divided every 12 hours
- CrCl 40-20 mL/min: 25% of normal daily dose divided every 12 hours
- CrCl 20-5 mL/min: 10% of normal daily dose every 24 hours
Administration Considerations
Route and Infusion
- IV bolus: Administer over 3-5 minutes 7
- Intermittent infusion: Dilute in 50-100 mL compatible solution 7
- IM injection: Acceptable but IV preferred for cellulitis
When to Hospitalize vs. Outpatient
- Outpatient: No SIRS, normal mental status, hemodynamically stable 1
- Hospitalize: SIRS present, altered mental status, hemodynamic instability, concern for deeper/necrotizing infection, immunocompromised, or outpatient treatment failing 1
Special Populations and Pitfalls
MRSA Coverage
If cellulitis is associated with penetrating trauma, known MRSA colonization/infection elsewhere, injection drug use, or SIRS, add vancomycin or another MRSA-active agent rather than using cefazolin alone 1.
Augmented Renal Clearance
In critically ill patients with augmented renal clearance (CrCl >130 mL/min/1.73m²), standard doses may be insufficient. Consider therapeutic drug monitoring if available 9.
Common Pitfalls to Avoid
- Underdosing obese patients: Don't use standard 1-2 g doses in patients ≥120 kg—use 3 g every 8 hours
- Forgetting renal adjustment: Always calculate CrCl and adjust accordingly; failure causes neurotoxicity
- Inadequate loading dose: Give full loading dose even in renal impairment before reducing maintenance doses
- Premature discontinuation: Treat minimum 5 days, extend if not improving