Calculating Oral Dose Equivalent to IV Dose with Different Dosing Intervals
The patient should be prescribed 50 mg of oral Drug X every 6 hours to maintain the same drug levels as 100 mg IV every 12 hours, given the 50% bioavailability.
Understanding the Problem
To determine the appropriate oral dose, we need to consider:
- The IV dose and frequency (100 mg every 12 hours)
- The oral bioavailability (F=0.5 or 50%)
- The new dosing interval (every 6 hours)
Step-by-Step Calculation
Step 1: Calculate the Total Daily IV Dose
- IV dose: 100 mg every 12 hours = 200 mg per day 1
Step 2: Calculate the Bioequivalent Oral Daily Dose
- With 50% bioavailability (F=0.5), the oral dose must be doubled to achieve the same blood levels 2
- Bioequivalent oral daily dose = 200 mg ÷ 0.5 = 400 mg per day 2
Step 3: Adjust for the New Dosing Frequency
- The oral medication will be given every 6 hours (4 times daily) 3
- Oral dose per administration = 400 mg ÷ 4 = 50 mg every 6 hours 3
Verification
To verify our answer:
- 50 mg oral × 4 times daily = 200 mg total oral absorbed drug (accounting for 50% bioavailability)
- This equals the 200 mg total daily IV dose 2
Important Considerations in Clinical Practice
- Bioavailability refers to the fraction of administered drug that reaches systemic circulation in unchanged form 4
- When converting from IV to oral formulations, bioavailability must be considered to maintain therapeutic drug levels 2
- Changing the dosing interval requires appropriate dose adjustment to maintain consistent drug levels 5
- More frequent dosing (every 6 hours vs. every 12 hours) allows for more stable drug concentrations throughout the day 3
Common Pitfalls to Avoid
- Failing to account for bioavailability when converting between IV and oral routes 2
- Not adjusting the dose when changing the dosing frequency 5
- Assuming linear pharmacokinetics without considering drug-specific properties 4
Therefore, the correct answer is B) 50 mg of oral Drug X every 6 hours.