Lexapro (escitalopram) and Prozac (fluoxetine) are both well-established SSRIs. For most adults with generalized anxiety, Lexapro is slightly more often the first choice because it's less activating in the early weeks. For adolescents and for OCD, fluoxetine is more often chosen because of its larger evidence base in those populations.
The short version
Both are effective SSRIs at standard doses. The two decisive differences are activation (fluoxetine is more activating, which can worsen anxiety in the early weeks for some patients) and half-life (fluoxetine's 4 to 6 day half-life and longer-acting metabolite produce a smoother long-term profile but slow down dose changes and switches). The Lexapro reference page and the Prozac reference page cover each drug in detail; this page focuses on the head-to-head comparison.
How they differ
Mechanism
Both block the serotonin transporter. Both produce their therapeutic effect over four to six weeks through receptor adaptation in anxiety circuitry. The mechanism is the same SSRI mechanism described in how CBT and SSRIs work.
Half-life
Lexapro has a half-life of about 30 hours. Steady-state is reached in about a week. Discontinuation produces symptoms over days unless a taper is used.
Fluoxetine has a half-life of 4 to 6 days for the parent drug and 1 to 2 weeks for its active metabolite norfluoxetine. Steady-state is reached over four to five weeks. Discontinuation symptoms are minimal because drug levels fall gradually over weeks.
This single difference shapes much of how each medication is used.
Activation
Fluoxetine is more activating than escitalopram. For patients with prominent fatigue and amotivation, the activation can be welcome. For patients with prominent anxiety, panic, or insomnia, fluoxetine can transiently worsen symptoms early in treatment.
Indications
Lexapro is FDA-approved for major depressive disorder and generalized anxiety disorder, with extensive off-label use across other anxiety conditions.
Fluoxetine is FDA-approved for major depressive disorder, OCD, bulimia nervosa, premenstrual dysphoric disorder, pediatric depression, and pediatric OCD. It's also widely used off-label for panic and generalized anxiety.
The MedlinePlus escitalopram page and the MedlinePlus fluoxetine page are good consumer-facing references.
Dosing
Lexapro: 10 to 20 mg daily; titration is straightforward and rapid.
Prozac: 20 to 40 mg daily for anxiety and depression; 40 to 80 mg for OCD. Dose changes are typically spaced four to six weeks apart because of the long half-life.
Drug interactions
Lexapro has a clean cytochrome P450 profile.
Fluoxetine inhibits CYP2D6 and CYP3A4 to a clinically meaningful degree, which can raise levels of certain other medications including some beta-blockers, certain antipsychotics, some opioids, and tamoxifen. The interaction profile matters more in patients on polypharmacy.
Use in adolescents
Fluoxetine has the largest pediatric evidence base of any SSRI and is FDA-approved for both pediatric depression and pediatric OCD. Many child and adolescent psychiatrists default to fluoxetine for that reason. The NIMH child and adolescent mental health page describes the broader treatment landscape. See also the pediatric anxiety overview.
When to choose Lexapro
- Adult with generalized anxiety as the primary presentation
- Adult with panic features where activation should be avoided
- Patient on polypharmacy where drug interactions matter
- Patient who prefers a quicker dose-adjustment timeline
When to choose Prozac
- Adolescent or young adult, particularly with OCD or depression
- Adult with OCD as the primary presentation
- Patient with prominent fatigue and amotivation where activation may help
- Patient with a history of discontinuation symptoms on other SSRIs
- Patient who is unreliable about taking medication every day
What both have in common
- Four to six week therapeutic timeline at a stable dose
- FDA boxed warning on increased suicidal ideation in patients under 25 in the first weeks
- Sexual side effects are common across both
- Not controlled substances; no addiction risk
For patients who don't respond to either, the typical next steps are an SNRI, augmentation, or reconsideration of the diagnosis. The NIMH anxiety disorders overview describes the broader treatment landscape.