Switching from Atorvastatin 20 mg to Simvastatin
When switching from atorvastatin 20 mg to simvastatin, use simvastatin 40 mg as the equivalent dose, as atorvastatin 20 mg provides approximately 43% LDL-C reduction while simvastatin 40 mg provides 41% reduction. 1
Dose Equivalency
- Atorvastatin 20 mg is classified as moderate-intensity statin therapy (achieving 30-49% LDL-C reduction), and the equivalent moderate-intensity dose of simvastatin is 40 mg daily 1
- Simvastatin 20 mg would provide less LDL-lowering than atorvastatin 20 mg and may result in inadequate lipid control 2
- Direct comparative trials demonstrate that atorvastatin 10 mg produces greater LDL-C reduction (37.1%) than simvastatin 20 mg (35.4%), confirming that simvastatin requires higher dosing for equivalent effect 2
Critical Drug Interaction Assessment Before Switching
You must evaluate for CYP3A4 inhibitors before prescribing simvastatin, as atorvastatin has fewer clinically significant interactions. This is the most important consideration when switching:
Mandatory Dose Restrictions with Simvastatin
- Limit simvastatin to 10 mg daily if patient takes diltiazem or dronedarone 3, 1
- Limit simvastatin to 20 mg daily if patient takes amiodarone, amlodipine, or ranolazine 3, 1
- Limit simvastatin to 20 mg daily if patient takes verapamil 1
When to Avoid Switching to Simvastatin
Do not switch to simvastatin if the patient requires high-intensity statin therapy (atorvastatin 40-80 mg equivalent), as simvastatin 80 mg is no longer recommended due to increased myopathy risk 1
Consider switching to rosuvastatin, pitavastatin, or pravastatin instead of simvastatin if the patient takes any CYP3A4 inhibitors listed above, as these alternative statins are not metabolized through CYP3A4 3, 1
Switching Protocol
Direct Switch Approach
- Discontinue atorvastatin 20 mg and start simvastatin 40 mg the next day - no washout period is required 4
- Schedule follow-up lipid panel in 4-6 weeks to assess efficacy 1
Monitoring Requirements After Switch
- Evaluate for muscle symptoms at 6-12 weeks after switching and at each subsequent visit 3
- Obtain creatine kinase measurement if patient reports any muscle pain, tenderness, or weakness 3
- Monitor ALT/AST at 12 weeks after the switch, then annually 3
- Monitor INR more closely if patient takes warfarin, though simvastatin-warfarin interaction is modest 1
High-Risk Populations Requiring Extra Caution
Exercise particular caution when switching in these patients:
- Elderly patients, especially thin or frail women 3
- Patients of Asian descent (higher risk of myopathy with simvastatin) 1, 3
- Patients with diabetes combined with chronic renal failure 3
- Patients with multisystem disease 3
Expected Efficacy Differences
- Simvastatin 40 mg produces slightly less LDL-C reduction than atorvastatin 20 mg in head-to-head trials (41% vs 43%) 2
- Simvastatin may produce greater HDL-C increases than atorvastatin at equivalent doses 5
- Triglyceride reduction is comparable between the two agents at moderate-intensity doses 6
Common Pitfall to Avoid
The most common error is switching to simvastatin 20 mg instead of 40 mg, which results in inadequate LDL-C lowering and failure to maintain therapeutic effect 1, 2. Always use simvastatin 40 mg when switching from atorvastatin 20 mg unless drug interactions mandate lower dosing.