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:

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