When switching from oral methotrexate to subcutaneous or intramuscular, by what percent should the dose be reduced due to higher bioavailability?

• A. 10%
• B. 50%
• C. 70%
• D. 30%

Answer: (D)

When the route changes from oral to subcutaneous or intramuscular, the amount reaching systemic circulation increases because the parenteral form bypasses the gut and is nearly fully bioavailable. Oral methotrexate has variable, often lower bioavailability (roughly around 70% on average). To avoid overdosing, its dose is typically reduced by about 30% when switching to the parenteral route. In practice, a weekly oral dose can be converted to roughly two-thirds of that amount when given subcutaneously or intramuscularly, with monitoring for efficacy and toxicity and adjustments as needed.