Dexamethasone to Prednisone Conversion
The conversion ratio from dexamethasone (Decadron) to prednisone is approximately 1:2.5, meaning 8 mg of dexamethasone is equivalent to 20 mg of prednisone. This conversion is directly supported by clinical practice data from the United States Cutaneous Lymphoma Consortium guidelines, which used "prednisone 20 mg or Decadron 8 mg/d × 5 d" interchangeably in treatment protocols 1.
Standard Conversion Ratio
- 8 mg dexamethasone = 20 mg prednisone (ratio of 1:2.5) 1
- This conversion reflects dexamethasone's approximately 6-7 times greater glucocorticoid potency compared to prednisone, but with a longer half-life that allows for less frequent dosing 2
- The longer half-life of dexamethasone (36-72 hours) compared to prednisone (12-36 hours) means dexamethasone provides more sustained anti-inflammatory effects 2
Clinical Application Examples
- When switching from prednisone 5 mg twice daily (10 mg total) to dexamethasone, use approximately 4 mg dexamethasone once daily 1
- For abiraterone therapy in prostate cancer, switching from prednisone 10 mg daily to dexamethasone 0.5-1 mg daily is recommended, though this represents a lower conversion ratio used specifically to minimize mineralocorticoid effects 1
- In acute asthma exacerbations, 16 mg dexamethasone daily for 2 days is equivalent to 50 mg prednisone daily for 5 days in terms of clinical efficacy 2
Important Pharmacokinetic Considerations
- Dexamethasone has minimal mineralocorticoid activity compared to prednisone, making direct milligram-to-milligram conversions based solely on anti-inflammatory potency potentially misleading in conditions where mineralocorticoid effects matter 1
- The prodrug form matters: dexamethasone phosphate converts rapidly and completely to active dexamethasone, while dexamethasone sulfobenzoate converts only 25% with a much longer half-life (5.4-7.4 hours), resulting in significantly reduced pharmacological response 3
- When using dexamethasone sulfobenzoate, expect only 50% of the lymphocyte suppression compared to dexamethasone phosphate at equivalent doses 3
Practical Conversion Guidelines
- For general anti-inflammatory purposes: Use 1 mg dexamethasone for every 2.5 mg prednisone 1
- For immune thrombocytopenia: Dexamethasone 40 mg daily × 4 days can be used as an alternative to prednisone 0.5-2 mg/kg/day, though the American Society of Hematology suggests prednisone over dexamethasone in children due to better tolerability 1
- For acute lymphoblastic leukemia: Dexamethasone 10 mg/m² per day is considered equivalent to prednisone 60 mg/m² per day (1:6 ratio), though this higher conversion ratio reflects the specific context of leukemia treatment where dexamethasone's superior CNS penetration is valued 4
Critical Caveats
- The conversion ratio varies by clinical context and should not be applied universally across all conditions 1, 4
- Dexamethasone causes significantly more toxicity in certain populations, particularly higher induction-related mortality in pediatric ALL (2.5% vs 0.9% with prednisone) 4
- Patients experience less vomiting with dexamethasone compared to prednisone (RR 0.29-0.32), which may improve adherence 5
- When converting between these agents, monitor closely for signs of over- or under-treatment, as individual patient responses vary significantly 6