What is the most common side effect of fluoxetine?

Fluoxetine, commonly known by its brand name Prozac, is a popular medication used to treat depression, anxiety disorders, and other mental health conditions. While it is generally well-tolerated, like any medication, it can come with side effects. In this text, we’ll explore the most common side effect of fluoxetine, backed up by studies from Google Scholar.

The most common side effect of fluoxetine: gastrointestinal symptoms

According to a study published in the Journal of Clinical Psychopharmacology, the most common side effect of fluoxetine is gastrointestinal symptoms, affecting up to 21% of patients. These symptoms can include nausea, vomiting, diarrhea, and constipation. The study also found that these symptoms were more likely to occur in the first few weeks of treatment and tended to decrease over time.

Another study published in the Journal of Affective Disorders found similar results, with gastrointestinal symptoms being the most common side effect of fluoxetine, affecting up to 30% of patients. The study also found that patients who experienced these symptoms were more likely to discontinue treatment.

Why do gastrointestinal symptoms occur?

The exact mechanism behind why fluoxetine can cause gastrointestinal symptoms isn’t fully understood. However, it is thought that the medication may affect the levels of serotonin in the gut, leading to changes in bowel function. Serotonin is a neurotransmitter that is involved in regulating mood, appetite, and digestion.

What can be done to manage gastrointestinal symptoms?

If you experience gastrointestinal symptoms while taking fluoxetine, there are several things you can do to manage them. Firstly, it’s important to take the medication with food to minimize stomach upset. Additionally, drinking plenty of fluids and eating a healthy, balanced diet can help alleviate symptoms. If your symptoms persist or are severe, you should speak to your doctor about adjusting your dosage or trying a different medication.

Other common side effects of fluoxetine

Yes, in addition to gastrointestinal symptoms, there are other common side effects of fluoxetine. Some of these include:

  1. Headache: According to a study published in the European Neuropsychopharmacology journal, headache is one of the most common side effects of fluoxetine, affecting up to 20% of patients.
  2. Insomnia: Fluoxetine can cause difficulty sleeping, which can lead to insomnia. This side effect may be more common in the first few weeks of treatment and tends to improve over time.
  3. Sexual dysfunction: Fluoxetine can also cause sexual side effects, including decreased libido, erectile dysfunction, and difficulty achieving orgasm. These side effects can be particularly distressing for some patients and may require a change in medication.
  4. Anxiety: In some cases, fluoxetine can cause an increase in anxiety symptoms, particularly in patients with pre-existing anxiety disorders. This side effect may be more likely to occur in the first few weeks of treatment.
  5. Weight changes: Fluoxetine can cause either weight gain or weight loss, depending on the individual. According to a study published in the Journal of Clinical Psychiatry, up to 25% of patients may experience weight changes while taking fluoxetine.

It’s important to note that not everyone will experience these side effects, and some patients may experience different side effects not listed here. If you’re taking fluoxetine and experience any new or concerning symptoms, it’s important to speak to your doctor. They can help determine if the symptoms are related to the medication or if there may be another underlying cause.

In conclusion, while fluoxetine is an effective medication for treating depression and anxiety, it can come with some side effects, with gastrointestinal symptoms being the most common. If you experience these symptoms, there are several strategies you can use to manage them. However, if they persist or are severe, it’s important to speak to your doctor about potential alternatives.


There are various forms of drugs available, such as tablets or liquids, and each may have a separate patient information leaflet (PIL) for different doses. It is important to refer to the PIL for the specific form and dose of the drug that you have been prescribed.

You can search for further information and PILs on websites such as:

  • The British National Formulary (BNF)
  • Electronic medicines compendium (emc)
  • The National Library of Medicine's DailyMed
  • The Food and Drug Administration (FDA) 
  • Different drug forms like tablets or liquids have specific patient information leaflets (PIL) for various doses. Refer to the PIL for your prescribed drug form and dose.

    Search for PILs on websites like:

    France

    • ANSM (Agence nationale de sécurité du médicament et des produits de santé): ansm.sante.fr

    Spain

    • AEMPS (Agencia Española de Medicamentos y Productos Sanitarios): aemps.gob.es

    Germany

    • BfArM (Bundesinstitut für Arzneimittel und Medizinprodukte): bfarm.de

    Italy

    For questions about your medication:

    • Consult your doctor, a healthcare professional, or a pharmacist

    France

    • Contact SOS Médecins: 3624
    • For mental health support, contact SOS Suicide: 01 45 39 40 00

    Belgium (French)

    Switzerland (French)

    • La Main Tendue: 143
    • Website: 143.ch

    Canada (French)

    • Centre de prévention du suicide du Québec: 1 866 APPELLE (1 866 277-3553)
    • Website: cpsquebec.ca

    Spain

    • Contact emergency number: 112
    • For mental health support, contact Teléfono de la Esperanza: 717 003 717

    Germany

    • Contact emergency number: 112
    • For mental health support, contact Telefonseelsorge: 0800 111 0 111 or 0800 111 0 222

    Italy

    • Contact emergency number: 112
    • For mental health support, contact Telefono Amico: 199 284 284

    Netherlands

    • Contact emergency number: 112
    • For mental health support, contact 113 Zelfmoordpreventie: 0800 0113

    Portugal

    • Contact emergency number: 112
    • For mental health support, contact Sos Voz Amiga: 21 354 45 45, 91 280 26 69, or 96 352 46 60

2 thoughts on “What is the most common side effect of fluoxetine?”

Comments are closed.

Log In

Forgot password?

Forgot password?

Enter your account data and we will send you a link to reset your password.

Your password reset link appears to be invalid or expired.

Log in

Privacy Policy

Add to Collection

No Collections

Here you'll find all collections you've created before.