How long does it take for Prozac to work?

In this article:

• How long does Prozac take to work?
• Interactions
• Dosage
• Contraindications
• Side effects

In this article

• How long does Prozac take to work?
• Interactions
• Dosage
• Contraindications
• Side effects

Following its rise in popularity during the 1990s and early 2000s, Prozac (fluoxetine) emerged as the most widely recognized antidepressant. It sparked the rise of numerous competing selective serotonin reuptake inhibitors (SSRIs). It gave hope to millions of people around the world with symptoms of depression, obsessive-compulsive disorder (OCD), anxiety disorders, premenstrual dysphoric disorder (PMDD), and bulimia nervosa. But although it may have seemed like a miracle pill, it isn’t an instant cure. 

Feeling the constant weight of a mental illness, many people might hope Prozac works like a pain medication—taking effect within an hour. While this desire for a fast fix is understandable, Prozac typically requires several weeks to improve mood. Here’s why.

How long does Prozac take to work? 

This depends, in part, on the person. “You may notice some improvements in anxiety and restlessness within the first week or two,” says Dr. Noah Kass, psychotherapist and founder at Kass Therapy & Hypnosis. But “it typically takes about four to six weeks to fully take effect, so patience is necessary.” One clinical study found that 55% of people taking Prozac saw some effect within two weeks. That percentage increased to around 80% after four weeks and nearly 90% after six weeks. Consider confiding in a close friend that you are starting this medication and asking them when your mood seems better. Oftentimes, others see a change in the patient before the patient can. 

There are a few theories as to why Prozac can take so long to work, but it likely has to do with the way it affects your levels of serotonin. It belongs to a class of drugs called selective serotonin reuptake inhibitors (SSRIs), and “a plausible and often-cited theory for how Prozac works is that it increases serotonin levels in the brain by preventing its reuptake, enhancing mood regulation,” Dr. Kass says.

Essentially, serotonin works as a neurotransmitter when it’s outside of our neurons. Prozac blocks the molecule responsible for transporting serotonin back into the cell after it’s released, leaving it active in the synapse. However, per Dr. Kass, “the brain needs time to adjust to the increased serotonin levels, involving changes in receptor sensitivity and neural circuits. Prozac also requires time to reach a stable concentration in the bloodstream.” Lower doses may also take longer to work than higher ones, and people with faster metabolisms may process the drug faster than others.

Once Prozac starts working, however, it “can help the person feel less nervousness, irritability and tension, and calmer. They may find they can concentrate better and have less racing thoughts,” according to Dr. Caroline Newton, MD, medical director at Mountainside Treatment Center. “Someone who is struggling with depression and taking Prozac might feel more energy, brighter mood, and more motivation and focus.”

What can affect how long it takes Prozac to work?

Even though it’s unclear exactly how Prozac functions in the brain, certain factors may influence its timeline. One of the most common is its dosage. For major depressive disorder (MDD) and OCD, the typical starting dose is 20 mg, although the dose may be increased if the patient doesn’t see any changes within a few weeks. While starting with a higher dose might seem tempting, this is rarely helpful because it may increase the risk of side effects.

Other factors that may influence Prozac’s efficiency include:

• Drug or alcohol use: These affect emotions and brain chemistry, potentially causing additional mood changes.
• New stressors: Introducing new circumstances that worsen anxiety or depression could make it more difficult for Prozac to compensate.
• Other medications: Prozac can interact with drugs like Monoamine oxidase inhibitors (MAOIs), antipsychotics, blood thinners, opioid pain relievers, and other medicines that may change how it functions.
• Liver disease: Since the liver metabolizes Prozac, “factors like liver function and other medical conditions can impact how the body tolerates the medication,” Dr. Kass says.

• Pregnancy: Being pregnant causes many bodily changes that may require a Prozac dosage alteration.
• Metabolism: “If someone has a quicker metabolism than others, they might notice the medication’s side effects at a quicker rate whether they are good or bad symptoms,” says Dr. Newton.
• Body weight: Obesity may hinder a drug’s absorption and influence how the body metabolizes it.

Whether you take Prozac in the morning or at night shouldn’t influence its effectiveness. Morning is ideal for most conditions since the drug may energize you. But evening might work better for depressive episodes from bipolar disorder and treatment-resistant depression. Regardless, the most important thing is to take it at the same time every day.

What is the standard Prozac dosage?

For most conditions, healthcare providers start patients at a 20 mg dosage. For bulimia nervosa, that starting dosage might be higher, and “if you are taking Prozac to manage OCD and PTSD symptoms, these can be more challenging to treat than other conditions and require a higher dose,” Dr. Newton says. But in most cases, healthcare providers won’t prescribe more than 80 mg per day. The dose is often titrated up, and frequent visits may be needed to discuss the effectiveness of the current dose and any side effects. According to the drug’s label, here are the ranges for adults with various mental health conditions:

• Major depressive disorder: 20–80 mg
• Obsessive-compulsive disorder: 20–60 mg
• Bulimia nervosa: 60 mg
• Panic disorder: 10–50 mg
• Bipolar I disorder: 20–50 mg taken with 5–12.5 mg of olanzapine

Of course, the specific dosage depends on the patient, their condition, and their unique circumstances. Only take the amount the healthcare provider prescribes since it’s possible to overdose on Prozac.

Who should avoid taking Prozac?

Prozac isn’t a perfect treatment option for everyone, particularly for people who are already taking certain medications. For example, combined with a rarely prescribed antihypertensive medication in the drug class of Monoamine oxidase inhibitors (MAOIs), Prozac can cause a life-threatening serotonin toxicity called serotonin syndrome. You shouldn’t take Prozac within 14 days of taking an MAOI like Nardil or Marplan. This risk applies to other SSRIs, too—like Lexapro, Zoloft, or Paxil (paroxetine)—since they can also increase the body’s serotonin levels.

Certain drugs may also cause an irregular heartbeat when taken simultaneously with Prozac. These drugs include pimozide, thioridazine, certain antipsychotics, and certain antifungals. Prozac can also interact with blood thinners, benzodiazepines, tricyclic antidepressants, opiate pain relievers, some antibiotics, and some NSAIDs. To be safe, it’s best to get medical advice from your healthcare provider about your current medications or supplements before starting Prozac treatment. Also, consider filling all your medications at the same pharmacy, as the pharmacist should also consider drug interactions and discuss them with the patient at the time of pick-up. 

Also, tell your healthcare provider if you have a history of the following medical conditions:

• History of heart conditions or stroke
• High blood pressure
• Liver or kidney problems
• Seizures
• Bipolar disorder

Additionally, Prozac may cause problems for newborn babies if the mother takes it later in the pregnancy. While most studies haven’t found issues with Prozac and breastfeeding, it can pass into the breast milk, and there are some reports of side effects in babies.

Lastly, Prozac may cause drowsiness in some people, so it’s often best to hold off on driving or operating machinery until you see if it affects you in this way.

Prozac side effects

Anyone considering Prozac should also be aware of its potential side effects. Just like the medication’s benefits, side effects might not appear for 2–4 weeks, per Dr. Newton. Not everyone will experience them, but according to MedlinePlus, some of the most common side effects of Prozac include:

• Nausea
• Diarrhea
• Trouble sleeping
• Nervousness
• Dry mouth
• Heartburn
• Weakness
• Shaking of a part of the body
• Loss of appetite and weight loss
• Unusual dreams
• Stuffy nose
• Lower sex drive
• Longer time to male ejaculation during intercourse (which may be a desired effect for those with premature ejaculation)
• Weakness
• Sweating
• Headache
• Difficulty concentrating 

Some of the rarer, more serious side effects include allergic reactions, suicidal thoughts, mania, abnormal bleeding, weight gain or loss, seizures, and serotonin syndrome. Contact a health professional immediately upon experiencing any of these.

SSRIs like Prozac also may cause withdrawal symptoms, especially in cases when a patient is taking a high dosage, has been taking the medication for a long time, or stops taking the medication abruptly. This is called antidepressant discontinuation syndrome and may manifest as insomnia, nausea, dizziness, lightheadedness, flu-like symptoms, or mood changes. In these cases, the healthcare provider may suggest tapering off instead.