Cephalexin 500 mg Dosing Instructions
Critical Dosing Error in Your Prescription
Your prescribed regimen of cephalexin 500 mg twice daily is INCORRECT and subtherapeutic for most bacterial infections. The FDA-approved and guideline-recommended dosing for cephalexin is 500 mg four times daily (every 6 hours) for standard adult infections, not twice daily 1, 2.
Correct Dosing Regimen
Standard Adult Dosing
- Take 500 mg orally four times daily (every 6 hours) for most bacterial infections 1, 2, 3
- For skin and soft tissue infections caused by methicillin-susceptible Staphylococcus aureus (MSSA), the dose is specifically 500 mg four times daily 2, 4, 3
- The typical duration is 7-10 days depending on clinical response 2, 3
Alternative Dosing (Limited Indications Only)
- 500 mg twice daily (every 12 hours) is FDA-approved ONLY for: streptococcal pharyngitis, uncomplicated skin infections, and uncomplicated cystitis in patients over 15 years 1
- This twice-daily regimen is NOT appropriate for moderate-to-severe infections or infections requiring more aggressive therapy 1
How to Take Cephalexin
- Take with 8 ounces of water to reduce gastrointestinal side effects 5
- Can be taken with or without food; taking with food may reduce stomach upset 5
- Space doses evenly throughout the day (e.g., 6 AM, 12 PM, 6 PM, 12 AM for four-times-daily dosing) 1
- Complete the entire course even if symptoms improve before finishing 3
Important Clinical Considerations
When to Expect Improvement
- Clinical response should be evident within 48-72 hours of starting therapy 3
- If no improvement occurs within 72 hours, contact your healthcare provider to consider resistant organisms or alternative diagnoses 3
Critical Limitations
- Cephalexin is completely ineffective against methicillin-resistant Staphylococcus aureus (MRSA) 4, 3
- If you have risk factors for MRSA (recent hospitalization, injection drug use, prior MRSA infection), cephalexin may not be appropriate 3
- Cephalexin is ineffective for Lyme disease and should never be used for this indication 6
Storage and Stability
- Store tablets at room temperature 1
- If using liquid suspension, refrigerate after mixing and discard after 14 days 1
Recommendation for Your Prescription
Contact your prescribing physician immediately to clarify the intended dosing regimen. If this prescription is for a standard bacterial skin infection, respiratory infection, or urinary tract infection requiring more than minimal therapy, the dose should likely be increased to 500 mg four times daily for optimal efficacy 1, 2, 3. The twice-daily regimen you were prescribed provides inadequate drug exposure for most infections and may lead to treatment failure 1.