• Most people feel lower levels of anxiety, restlessness and tiredness when Prozac first starts to work. Your sleep, energy and appetite may improve over the first month and you may have a better focus on daily tasks.
• Some people may start to feel results in 1 to 2 weeks, while others may take up to 8 weeks or longer to feel the effects. A depressed mood can take 6 to 8 weeks to fully respond to treatment.

Side effects, like trouble sleeping, agitation, headache or stomach upset are usually temporary and will often subside within the first month of treatment.

Results are individual, but call your healthcare provider if you have side effects that make you want to stop treatment. Do not stop taking Prozac without speaking to your doctor first.

How soon will I notice Prozac working?

Clinical research has shown that over half of patients (55.5%) who respond to Prozac (fluoxetine) for depression will start responding by the second week of treatment.

• Researchers evaluated the time until the start of an antidepressant sustained response with fluoxetine (Prozac) treatment given at 20 mg/day in an 8-week study of 182 patients with major depression.
• Patients were assessed using the 17-item Hamilton Depression Rating Scale every 2 weeks, a common measure used in depression studies. A sustained response was defined as a 30% decrease on the depression scale from the start of the study which led to a 50% reduction by week 8.
• Over half of the patients (55.5%) responded by week two, 24.7% by week five and 9.3% (35 patients) by week six. By week six, the probability of having a response to Prozac treatment (20 mg/day) was 90%.
• However, in patients without a response to Prozac by week 4 to 6, there was a 73% to 88% chance there would be no response by week 8.

Is Prozac an SSRI?

Yes, Prozac (generic name: fluoxetine) is in the selective serotonin reuptake (SSRI) class of antidepressants. It works by increasing levels of a chemical (neurotransmitter) in your brain called serotonin.

• Prozac is approved by the FDA to treat major depressive disorder, obsessive compulsive disorder (OCD), panic disorder, bulimia nervosa (an eating disorder), and premenstrual dysphoric disorder (PMDD).
• It is a commonly used, widely available and affordable SSRI treatment. It is available as a generic option in the U.S. and a month's supply of fluoxetine usually costs less than $10.00.
• It comes an oral capsule and is taken once per day in the morning.

What side effects occur in the first week with Prozac?

All selective serotonin reuptake inhibitors are associated with various side effects, many of which are similar. You probably will not experience all of the side effects reported with Prozac.

The most common side effects with Prozac (in at least 10% of patients) include:

• anorexia (loss of appetite)
• anxiety
• diarrhea
• dry mouth (xerostomia)
• headache
• insomnia (trouble sleeping)
• nausea
• nervousness
• sexual dysfunction
• sore throat (pharyngitis)
• tremor
• weakness
• yawning

Prozac is frequently associated with insomnia, headache, agitation and sexual dysfunction. Many of these side effects will go away within a week or two. However, sexual side effects (such as decreased libido, lack of orgasm, delay in ejaculation, impotence) can be an ongoing side effect and has been reported in up to 50% of patients.

Weight gain is not a common side effect with Prozac treatment, and some patients may lose weight, although this can be temporary.

Speak to your healthcare provider about any side effects that do not subside over time, as dosage adjustments or other treatment options may be possible.

Related: Prozac side effects (in more detail)

Bottom Line

• Within the first one to two weeks, most people feel lower levels of anxiety, restlessness or tiredness with Prozac treatment. Your sleep, energy and appetite may improve over the first month and you may have a better focus on daily tasks. A depressed mood can take up to 8 weeks to fully respond.
• Research has shown that over half of patients (55.5%) who respond to Prozac treatment for depression will start responding by the second week.
• Prozac can be an “activating” antidepressant and can lead to side effects in the first few weeks that may impact your response, such as insomnia (trouble sleeping), anxiety or nervousness. In most people, these side effects will start to subside in a few weeks.
• Sexual side effects can be ongoing if not addressed by your healthcare provider.