Switching From Prozac To Paxil

In our latest question and answer, the pharmacist discusses what to expect when switching from Prozac (fluoxetine) to Paxil (paroxetine).


What can I expect when switching from 20mg of Prozac to Paxil?

Asked by Chuck On May 18, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On May 21, 2018

Doctor Writing RxSwitching from one selective serotonin reuptake inhibitor (SSRI) to another may help if the one being used is not effective, or if it is being done to minimize certain side effects. Both fluoxetine (Prozac) and paroxetine (Paxil) are SSRI medications.

Resources show that fluoxetine 20 mg is equivalent to a dose of paroxetine 20 mg; however, the dose of paroxetine may need to be increased after the starting at the initial strength of 20 mg depending on how effective it is. SSRIs can take 4-8 weeks to feel maximum effectiveness.

How To Switch From Prozac To Paxil

When switching from fluoxetine to another SSRI, it is generally recommended to:

  • Taper down to a low dose of fluoxetine then stop for 7 days before starting another SSRI. This is to allow the drug to be metabolized and excreted from the body and to avoid the potential for excessive serotonin.
  • However, be sure to follow your doctors advice and protocol as they have your complete medical history.

Although many of the warnings, cautions, contraindications, and drug interactions are the same, side effects may be more or less commonly experienced depending on the SSRI.

Side Effects

According to the prescribing information, common side effects of fluoxetine can include:

  • Nausea
  • Sleepiness, lack of energy, or insomnia
  • Abnormal dreams
  • Anorexia
  • Dry mouth
  • Diarrhea
  • Sweating
  • Tremor
  • Nervousness
  • Decreased sex drive or ejaculatory issues

According to the prescribing information for paroxetine, common side effects can include:

  • Nausea
  • Sleepiness, lack of energy, or insomnia
  • Dizziness
  • Headache
  • Dry mouth
  • Constipation or diarrhea
  • Sweating
  • Tremor
  • Decreased sex drive or ejaculatory issues

Paxil And Prozac Side Effect Comparison

Studies have shown some side effects are experienced more or less commonly in certain antidepressant medications.

More common with fluoxetineMore common with paroxetineSame potential


Loss of appetite



Dry mouth



Lack of energy


Ejaculation issues





Decreased sex drive

Warnings and Cautions of SSRIs

  • Potential for worsening depression and suicide risk, especially for people ages 18-24 years old with depression or other psychiatric disorders
  • People should be screened for bipolar disorder before use, as an antidepressant alone may cause hypomanic or manic episodes
  • Risk of serotonin syndrome or neuroleptic malignant syndrome-like reactions (NMS), especially with concomitant use with other medications that could increase serotonin levels
    • Medications affecting serotonin
      • Medications that can increase serotonin levels: tryptophan, St. John's Wort, tramadol, buspirone, lithium, fentanyl, tricyclic antidepressants, triptans
      • Medications that decrease metabolism of serotonin and may cause build up: monoamine oxidase inhibitors (MAOIs), linezolid, methylene blue
    • Symptoms of serotonin syndrome or NMS
      • Agitation
      • Hallucinations
      • Rapid heartbeat
      • Blood pressure fluctuations
      • Fever
      • Severe nausea, vomiting, diarrhea
      • Muscle rigidity
  • May cause weight loss, decreased sodium levels, abnormal bleeding (increased risk with anticoagulants, aspirin, nonsteroidal anti-inflammatory drugs)
  • Should only be used if benefits outweigh risks during pregnancy
    • Paroxetine has shown a potential to cause defects when taken during pregnancy

Contraindications of SSRIs

  • Use with monoamine oxidase inhibitors (MAOIs) or using paroxetine or MAOIs within 14 days of one another, including linezolid or methylene blue
  • Concomitant use with thioridazine or pimozide
  • Use with a hypersensitivity to the medication or any inactive ingredients


  • Do not stop taking SSRIs abruptly
    • Doses typically need tapering down in order to avoid side effects of stopping the medication
    • Potential effects of stopping the medication include: mood changes (irritability, agitation, anxiety), confusion, headache, insomnia, dizziness, and lethargy

About the Pharmacist

Dr. Anna Staudt Pharm.D

Dr. Anna Staudt graduated in 2017 with a Doctor of Pharmacy degree. She is completing a pharmacy fellowship focused on ambulatory care. Her current practice site is at a Federally Qualified Health Center (FQHC) where she deals with the underserved.

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