Can I start Prozac (fluoxetine) 10 mg daily in a patient who has been taking Lexapro (escitalopram) 10 mg daily for 3 days?

Switching from Lexapro to Prozac: Safety and Protocol

No, you should not start Prozac 10 mg daily while still taking Lexapro. A washout period is required between these two SSRIs to avoid serotonin syndrome. 1

Rationale for Washout Period

When switching between SSRIs, proper medication management is essential to prevent potentially dangerous drug interactions:

• Both escitalopram (Lexapro) and fluoxetine (Prozac) are selective serotonin reuptake inhibitors that increase serotonin availability in the brain 2
• Taking both medications simultaneously significantly increases the risk of serotonin syndrome, a potentially life-threatening condition
• Fluoxetine has an exceptionally long half-life of 4-6 days, with its active metabolite norfluoxetine having a half-life of 4-16 days 3
• Escitalopram has a shorter half-life of approximately 27-33 hours 4

Proper Switching Protocol

The appropriate protocol for switching from Lexapro to Prozac is:

1. Discontinue Lexapro completely
2. Wait for a washout period of at least 5 half-lives (approximately 5-7 days for escitalopram) 2
3. Begin Prozac at 10 mg daily after the washout period is complete

Important Monitoring Considerations

When switching antidepressants, careful monitoring is essential:

• Watch for signs of serotonin syndrome (agitation, tremor, rapid heart rate, elevated temperature, muscle rigidity)
• Monitor for behavioral activation/agitation, especially in the first months of treatment 1
• Be aware that fluoxetine's long half-life means it takes 7-10 days to reach steady-state concentrations 4
• The FDA has a boxed warning for increased suicidality risk with antidepressants, particularly in adolescents and young adults up to age 24 2, 1

Special Considerations

• Fluoxetine's long half-life provides some protection against missed doses but requires longer washout periods when switching to other medications
• The optimal duration for an initial SSRI trial is 4-8 weeks to determine efficacy 1
• If the patient has been on Lexapro for only 3 days, consider whether continuing Lexapro might be more appropriate than switching, as therapeutic effects typically take several weeks to manifest

Common Pitfalls to Avoid

1. Concurrent administration: Never administer two SSRIs simultaneously due to the high risk of serotonin syndrome
2. Inadequate washout: Failing to allow sufficient time between medications can lead to adverse effects
3. Insufficient monitoring: Close follow-up is essential during medication transitions
4. Premature efficacy assessment: Remember that antidepressants typically take 4-8 weeks for full therapeutic effect 1

By following these guidelines, you can safely transition your patient from Lexapro to Prozac while minimizing risks and optimizing treatment outcomes.