What is the suitable dose of Keflex (Cephalexin) for a 19-year-old patient weighing 185 pounds?

Keflex (Cephalexin) Dosing for a 19-Year-Old Weighing 185 lbs

For a 19-year-old patient weighing 185 pounds (84 kg), the appropriate dose of Keflex (cephalexin) is 500 mg orally four times daily or 1 gram twice daily for most infections.

Dosing Considerations

The dosing of cephalexin depends on several factors:

1. Standard Adult Dosing:

   • The Infectious Diseases Society of America (IDSA) guidelines recommend 500 mg four times daily or 1 gram twice daily for adults 1
   • For skin and soft tissue infections, 500 mg every 6 hours is the standard adult dose 1

2. Weight-Based Considerations:

   • At 185 lbs (84 kg), this patient is considered an adult for dosing purposes
   • The patient's weight does not necessitate dose adjustment within normal adult parameters

3. Specific Infection Types:

   • For Group A Streptococcal pharyngitis: 20 mg/kg twice daily (maximum 500 mg per dose) for 10 days 1
   • For skin and soft tissue infections: 500 mg four times daily 1
   • For urinary tract infections: 1 gram twice daily for 10 days has shown equivalent efficacy to four-times-daily dosing 2, 3

Dosing Schedule Options

1. Four-times-daily regimen:

   • 500 mg orally every 6 hours
   • Provides more consistent drug levels throughout the day
   • May be preferred for more severe infections

2. Twice-daily regimen:

   • 1 gram orally every 12 hours
   • Improves compliance
   • Studies have shown equivalent efficacy to four-times-daily dosing 2, 4

Duration of Treatment

• Standard duration is 7-10 days for most infections
• For Group A Streptococcal pharyngitis, a full 10-day course is recommended 1
• For uncomplicated urinary tract infections, shorter courses may be effective

Special Considerations

• Renal Function: No dose adjustment is needed for normal renal function. If creatinine clearance is <30 mL/min, dose reduction would be required 5
• Absorption: Cephalexin is rapidly absorbed in the upper intestine and not affected by food 5
• Distribution: Achieves good tissue penetration except in cerebrospinal fluid 5
• Common Side Effects: Gastrointestinal symptoms (diarrhea, nausea), hypersensitivity reactions

Common Pitfalls to Avoid

1. Underdosing: Using pediatric doses for adult-sized patients can lead to treatment failure
2. Inadequate frequency: Twice-daily dosing should use the higher 1 gram dose, not the 500 mg dose
3. Inappropriate duration: Not completing the full prescribed course can lead to treatment failure and resistance
4. Ignoring allergies: Cross-reactivity with penicillin allergies can occur in approximately 10% of patients

For this 19-year-old patient weighing 185 pounds with no mentioned comorbidities, either 500 mg four times daily or 1 gram twice daily would be appropriate, with the twice-daily regimen potentially offering better adherence.