For an 81.4 kg patient, is dosing 300 mg every 8 hours appropriate to achieve a target of 10 mg/kg/day?

Dosing 300mg Every 8 Hours: Calculation and Appropriateness

No, 300 mg every 8 hours is not appropriate for this patient. This regimen would deliver approximately 11 mg/kg/day (900 mg total daily dose ÷ 81.4 kg = 11.05 mg/kg/day), which exceeds the target of 10 mg/kg/day.

Correct Dose Calculation

To achieve exactly 10 mg/kg/day for an 81.4 kg patient:

• Target total daily dose: 81.4 kg × 10 mg/kg = 814 mg/day
• Divided every 8 hours (3 doses): 814 mg ÷ 3 = 271 mg per dose
• Practical dosing: Round to 275 mg every 8 hours (825 mg/day = 10.1 mg/kg/day)

Why 300mg Every 8 Hours is Inappropriate

The proposed regimen delivers 900 mg daily, representing a 10.5% overdose compared to the weight-based target. While this may seem like a small difference, medication dosing should adhere to calculated weight-based targets, particularly when:

• The medication has a narrow therapeutic index
• Dose-dependent toxicities are a concern
• Renal or hepatic clearance is involved
• The patient has comorbidities affecting drug metabolism

Alternative Dosing Strategies

If 275 mg per dose is not feasible due to available formulations:

• Option 1: 250 mg every 8 hours (750 mg/day = 9.2 mg/kg/day) - slightly under target but safer
• Option 2: 300 mg every 8 hours with dose reduction after monitoring drug levels or clinical response
• Option 3: Adjust dosing interval rather than dose amount (e.g., 250 mg every 6 hours if more frequent dosing is acceptable)

Critical Monitoring Considerations

Regardless of the final dosing decision, implement these safeguards:

• Baseline assessment: Obtain renal function (creatinine, eGFR), hepatic function (AST, ALT), and relevant drug levels before initiating therapy
• Therapeutic drug monitoring: Check trough levels after steady state is achieved (typically 3-5 half-lives)
• Clinical response monitoring: Assess efficacy markers and adverse effects at regular intervals
• Dose adjustment triggers: Reduce dose if toxicity develops, renal function declines, or drug levels exceed therapeutic range

Common Pitfalls to Avoid

• Do not round up to convenient doses without recalculating the actual mg/kg/day delivered
• Do not assume "close enough" when weight-based dosing is specified - calculate precisely
• Do not ignore cumulative effects - even small daily overdoses accumulate over time
• Do not proceed without verifying available formulations and whether dose splitting or compounding is necessary