Switching from Atorvastatin 20 mg to Simvastatin
Switch directly to simvastatin 40 mg daily, as this provides equivalent LDL-C reduction (approximately 41-43%) to atorvastatin 20 mg and maintains moderate-intensity statin therapy. 1
Equivalent Dosing
- Atorvastatin 20 mg and simvastatin 40 mg are both classified as moderate-intensity statin therapy, achieving 30-49% LDL-C reduction 1
- The American Diabetes Association specifically recommends simvastatin 40 mg as the equivalent dose when switching from atorvastatin 20 mg 1
- Do not use simvastatin 20 mg—this is the most common prescribing error and results in inadequate LDL-C lowering and loss of therapeutic effect 1
Mandatory Pre-Switch Assessment for Drug Interactions
Before prescribing simvastatin, you must evaluate for CYP3A4 inhibitors, as simvastatin has significantly more clinically important drug interactions than atorvastatin 1:
Required Dose Restrictions with Simvastatin:
- Limit to 10 mg daily if patient takes diltiazem or dronedarone 1
- Limit to 20 mg daily if patient takes amiodarone, amlodipine, ranolazine, or verapamil 1
- These restrictions are mandatory due to increased myopathy risk 1
When NOT to Switch:
- Avoid switching to simvastatin entirely if the patient requires high-intensity statin therapy, as simvastatin 80 mg is no longer recommended due to unacceptable myopathy risk 1
Switching Protocol
- Perform a direct switch—stop atorvastatin and start simvastatin 40 mg the next day 1
- Schedule follow-up lipid panel in 4-6 weeks to confirm therapeutic equivalence 1
Post-Switch Monitoring Requirements
Muscle Safety Monitoring:
- Evaluate for muscle symptoms (pain, tenderness, weakness) at 6-12 weeks after switching and at every subsequent visit 1
- Obtain creatine kinase measurement if any muscle symptoms are reported 1
Laboratory Monitoring:
- Monitor ALT/AST at 12 weeks after the switch, then annually 1
- If patient takes warfarin, monitor INR more closely (though simvastatin-warfarin interaction is modest) 1
High-Risk Populations Requiring Extra Caution
Exercise particular caution when switching in 1:
- Elderly patients, especially thin or frail women
- Patients of Asian descent
- Patients with diabetes combined with chronic renal failure
- Patients with multisystem disease
Clinical Context
While both statins are generally well-tolerated with similar adverse event rates 2, simvastatin's extensive CYP3A4 metabolism creates more potential for drug interactions compared to atorvastatin 1. Research demonstrates that atorvastatin may provide slightly greater LDL-C reduction at equivalent doses 3, but simvastatin 40 mg remains the guideline-recommended equivalent for moderate-intensity therapy 1.