How Long Does it Take for Prozac to Work?

By Terez Malka, MD
Medically reviewed checkmarkMedically reviewed
March 14, 2022

Prozac is a commonly prescribed antidepressant medication.

It is approved by the U.S. Food and Drug Administration (FDA) to treat symptoms of depression, and is also used to treat anxiety.

Depression impacts 280 million people around the world, according to the World Health Organization (WHO).

If you are considering an antidepressant, it’s natural to wonder how long it might take to help you feel better.

In this article, I’ll explain how Prozac works, and how long it takes to start taking effect.

I’ll also outline some of the medication’s side effects, what you should know before taking it, and who should not take Prozac.

I’ll also talk about how to know when you are feeling better, and when to talk to your doctor.

How Long Does it Take for Prozac to Work?

Prozac helps the brain increase its levels of serotonin, a brain chemical that helps to balance mood.

Prozac does not have a rapid or immediate effect on mood, but once it does achieve consistent levels in the body, it is a highly effective medication.

Many patients see some improvement on Prozac right away, but most people notice the antidepressant effects after 2-4 weeks of daily use.

Prozac has a longer half-life than many other antidepressants at 2-4 days, which means it remains in your body longer than some other medications.

The half-life is the amount of time it takes for a medication to decrease in your body by 50%.

This long half-life makes it easier to stop Prozac than some other antidepressants, which can cause withdrawal symptoms when you stop them abruptly. 

The metabolite, or substance, that Prozac is broken down into has an even longer half-life of 7-9 days, which means it may stay in your system for a few weeks after you stop taking it.

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Prozac Side Effects

Prozac may cause some common side effects, including:

  • Mood changes: While Prozac is often used to treat anxiety and is approved by the Food and Drug Administration (FDA) for the treatment of panic disorder, some patients may experience nervousness or anxious feelings when they start taking it. These usually subside as your body adjusts to the medication. Always let your healthcare provider know if side effects are concerning.
  • Sleep changes: Problems falling or staying asleep, or fatigue.
  • Gastrointestinal symptoms: The most common side effects of Prozac include nausea, diarrhea, and heartburn. Loss of appetite is possible, but is not as common. If you lose your appetite or struggle to eat while taking Prozac, especially if you are younger than 25, tell your provider.
  • Oral symptoms: Dry mouth is a common antidepressant symptom, as is yawning. Both can be common with Prozac, or with the generic version, fluoxetine.
  • Changes in sex drive or sexual ability: People with penises may experience ejaculation problems, whereas people with vaginas are more likely to experience decreased libido.

What Conditions Does Prozac Help With?

Prozac (fluoxetine) is approved by the FDA for the treatment of the following:

  • Major depressive disorder (MDD) (also known as clinical depression)
  • Obsessive-compulsive disorder (OCD)
  • Panic disorder
  • Bulimia
  • Binge eating disorder
  • Premenstrual dysphoric disorder (PMDD)
  • Bipolar disorder (along with olanzapine)
  • Treatment-resistant depression (along with olanzapine)

Doctors may also use Prozac for off-label purposes. “Off-label” means that the medication has not been approved by the FDA for the condition the doctor is prescribing it for, but the doctor can provide justification for why the medication can help manage symptoms.

Some common off-label uses of Prozac include: 

Prozac is a selective serotonin reuptake inhibitor, or SSRI. Serotonin is a neurotransmitter, a chemical used by the nervous system to transmit messages in the body and brain.

Usually, when it’s finished relaying the message, the brain clears, or “reuptakes,” serotonin.

Prozac and other SSRIs work by blocking the clearance of serotonin from the brain.

Because of this increase in the brain’s access to this important neurotransmitter, Prozac can help stabilize mood, support feelings of calm, improve the perception of well-being, and increase feelings of positivity.

What to Know Before Taking Prozac

Prozac may cause serious interactions with other medications.

If you take any of the following, tell your doctor before starting Prozac:

  • MAOIs (monoamine oxidase inhibitors)
  • TCAs (tricyclic antidepressants)
  • Benzodiazepines
  • Antipsychotics
  • Anticonvulsants
  • Serotonergic drugs
  • Anticoagulant drugs
  • NSAIDs
  • Aspirin
  • Olanzapine
  • Thioridazine
  • Pimozide

This is not a complete list of potential interactions.

Prozac should not be consumed with alcohol, marijuana, CBD, or any illicit drugs, since they can have serious interactions and change the way that the brain responds to serotonin.

Tell your doctor about everything you take, including other medications, supplements, vitamins, and illicit drugs.

Who should and should not take Prozac

In addition to medication interactions, Prozac is contraindicated in certain people or may be riskier for certain populations.

  • People who are pregnant: Prozac should only be used in pregnant people if the benefit outweighs the potential risk. It is a Class C pregnancy drug. There are no studies assessing the safety of Prozac in the first trimester of pregnancy, and it is not fully understood how it impacts labor and delivery. If you become pregnant while taking Prozac, tell your provider right away.
  • People who are breastfeeding: Prozac is excreted in breastmilk, so nursing people should not take it.
  • Older adults: Prozac stays in the system longer than other medications. This can lead to higher concentrations of the drug in the body, which may increase the risk for hyponatremia (low sodium levels). 
  • Liver problems: Prozac is processed by the liver. Decreased liver function increases the half-life, leading to higher levels of the drug.

How to Know When You Are Getting Better

Prozac is very effective for the treatment of depression and other psychiatric conditions, but every person responds differently to medications. 

It takes an average of 2-4 weeks before your body acclimates to Prozac and before it is circulating effectively in your system.

You may not notice improvements until it has been that long, and as you initially adjust to the medicine, you may notice some side effects that could make things feel worse.

Keep your doctor or provider informed about how you are feeling.

If the dose is right, you should start to feel better within 2-4 weeks, but you may continue to improve over more weeks or even months if you are taking Prozac longer-term.

It often takes a few dose adjustments to find the perfect long-term dose. 

How you define getting better may be different from someone else.

To know if you are improving, ask yourself the following questions:

  • Am I able to function in my daily life better than before I started Prozac (for example, keeping up with household responsibilities, caregiving, self-care, hygiene)?
  • Am I able to perform better than before I started Prozac in work, school, or other pursuits or interests (for example, are focus and motivation better)?
  • Am I excited about things that I care about and enjoy more than before I started taking Prozac?
  • Do I feel more confident socially, emotionally, or in other ways than before I started taking Prozac?

There are many ways to decide if Prozac is working for you and supporting your well-being.

The most important way is if you are feeling better and experiencing an improved quality of life.

When to See a Doctor

If you are worried about depression or another mental health condition, talk to a doctor or mental health professional.

If you experience any concerning side effects or are feeling worse while on Prozac, let your provider know.

If you believe you are experiencing a severe allergic reaction are are having suicidal thoughts, seek emergency medical care or call 9-1-1. You can also get free 24/7 support from a suicide and crisis expert by calling or texting 988. If you’d prefer to chat online, you can chat with a suicide and crisis expert by visiting the Lifeline Chat.

If you have been taking Prozac for 4 weeks but you are not feeling better, let your prescriber know.

You may need a higher dose, or Prozac may not be an effective drug for you.

If your provider does switch your medication, you may need to taper or wean off of Prozac before starting something else.

Be sure to follow your healthcare provider’s instructions carefully.

Never stop taking Prozac or any other antidepressant suddenly without your doctor’s guidance.

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How K Health Can Help

Think you might need a prescription for Prozac (fluoxetine)?

K Health has clinicians standing by 24/7 to evaluate your symptoms and determine if Prozac is right for you.

Get started with our free assessment, which will tell you in minutes if treatment could be a good fit. If yes, we’ll connect you right to a clinician who can prescribe medication and have it shipped right to your door.

Frequently Asked Questions

What does Prozac feel like when it starts working?
Initially, Prozac may lead to some gastrointestinal symptoms or other side effects that may make you question your decision to take it. However, as your body adjusts to the medication, these symptoms should lessen. As your body reaches higher levels of the medication, typically over 2-4 weeks, Prozac may help your mood feel lighter, and you may feel calmer, happier, less anxious, more relaxed, or more able to function normally.
What is the peak time for Prozac?
Peak plasma concentration of Prozac occurs 6-8 hours after taking the drug. It works best when taken every day for 2-4 weeks. Prozac has a long half-life of 2-4 days, meaning that it takes that long for the dose in your body to be reduced by half. As Prozac is broken down, the resulting substances, called metabolites, stay in your system even longer—at least 7-9 days before half of these substances are gone.
K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Terez Malka, MD

Dr. Terez Malka is a board-certified pediatrician and emergency medicine physician.

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