Fluoxetine (Prozac): dosage, uses, side effects

Most people have heard of Prozac (generic name: fluoxetine), a drug that has been used to treat depression since the 1980s. But what do you really know about it? Read on to learn about fluoxetine’s uses, side effects, and more.  

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Studies show that depression rates seem to be rising, especially with the COVID-19 pandemic (Czeisler, 2020). Until the late 1980s, treatment options for depression were limited and included medications like tricyclic antidepressants (TCAs) and monoamine oxidase inhibitors (MAOIs), which can have significant side effects. Since then, the number of treatment options has increased. 

When fluoxetine hydrochloride (brand name Prozac), the first selective serotonin reuptake inhibitor (SSRI), was introduced in 1988, it took off—in large part because side effects were so mild compared to the previously available options. Drug companies took notice and decided to develop other SSRI drugs, and just like that, the reign of SSRIs over mental health treatment began. Other SSRIs include sertraline, citalopram, and escitalopram (Chu, 2021). 

Today, about one in every eight adults in the United States currently takes antidepressants, and SSRIs are some of the first-line psychiatric medications prescribed (Brody, 2020; Chu, 2021). We know that fluoxetine and other SSRIs work by increasing serotonin in the brain, but they don’t do it by increasing the amount produced. Instead, they block our body’s natural “reuptake,” or recycling, of the neurotransmitter. The result is more serotonin around our brain cells for a longer period of time (Sohel, 2020). 

Studies have shown that people with depression have lower levels of serotonin in their bodies. While researchers aren’t entirely sure how low serotonin contributes to the development of conditions like depression, clinical trials have proven that SSRIs are an effective treatment option for people with depression, as well as other conditions (Jakubovski, 2018). 

Fluoxetine is approved by the U.S. Food and Drug Administration (FDA) for the treatment of:

• Major depressive disorder (unipolar depression) is a condition characterized by symptoms of persistently sad moods, a lack of desire to do anything, a sense of hopelessness or guilt, low energy, poor appetite, and trouble concentrating that disrupt a person’s ability to function (Bains, 2021).

• Obsessive-compulsive disorder (OCD) isn’t just about wanting things to be “just-so.” It’s a serious condition associated with intrusive thoughts and feelings of discomfort that a person typically alleviates by using ritualized behaviors like excessive hand washing or repetitively flipping the light switch on and off (Brock, 2020). 

• Bulimia nervosa is an eating disorder in which people consume large quantities of food in a short period of time (binge eating) and then purge by making themselves vomit, using medications like laxatives or diuretics, or exercising excessively to get rid of the extra calories they’ve consumed (Jain, 2020). 

• Panic disorder is an often debilitating disorder characterized by sudden attacks of intense anxiety and fear associated with rapid heartbeat, sweating, nausea, chest pain, and difficulty breathing (panic attacks) that appear in the absence of any real danger or cause (Cackovic, 2020). 

• Bipolar major depression occurs in bipolar disorder, a condition characterized by periods of extreme highs (feelings of euphoria, energy, reduced need for sleep) called “manic episodes,” followed by periods of extreme lows (inability to function, inability to get out of bed, pervasive sadness, etc.). Fluoxetine is approved for bipolar depression when combined with olanzapine (Jain, 2021).

• Treatment-resistant depression is depression that is not getting better after trying two different antidepressants. Fluoxetine is approved for this condition when combined with olanzapine (FDA, 2017).

Other uses for fluoxetine

In the year 2000, the company that first patented fluoxetine and sold it as Prozac, Eli Lily, was about to lose its patent. So they invested a significant amount of resources into having the medication re-labeled by the FDA to treat premenstrual dysphoric disorder (PMDD), a condition characterized by significant irritability, anxiety, or depression in the time before you get your period.

Their plan worked, and they received FDA approval for the new use, allowing them to relabel the drug Sarafem and sell it for more money (Vedantam, 2001). So what’s the difference between Sarafem and Prozac? Nothing.

Healthcare providers sometimes use medications for conditions not specifically approved by the FDA. These “off-label” treatments are often widely studied. Fluoxetine has been used off-label in the treatment of a wide range of conditions, including (UptoDate, n.d.):

• Binge eating disorder

• Body dysmorphic disorder

• Fibromyalgia that doesn’t respond to other treatments

• Generalized anxiety disorder (GAD)

• Post-traumatic stress disorder (PTSD)

• Premature ejaculation 

• Selective mutism

• Social anxiety disorder 

Fluoxetine carries a black box warning, which is the strongest warning the FDA can give, because some people taking this medication can develop worsening depression and suicidal thoughts or tendencies. This is a rare side effect but is more common in kids, teenagers, and young adults up to the age of 24. The FDA warns that a person taking fluoxetine (or other antidepressant medications) may have an increased risk of suicide compared to a person not taking these medications (FDA, 2017). 

Common side effects people taking fluoxetine may experience include (UptoDate, n.d.):

• Headache

• Sexual dysfunction 

• Trouble sleeping 

• Anxiety or nervousness

• Drowsiness

• Digestive problems like nausea, heartburn, or diarrhea

• Dry mouth

• Uncontrollable shaking

• Loss of appetite and weight loss

• Changes in sex drive or ability (including increased erectile dysfunction)

• Sweating

• Cognitive issues like difficulty concentrating or memory problems

Side effects usually appear within the first week after starting fluoxetine, and the frequency of adverse effects increases with higher doses. This list is not exhaustive. Speak with your healthcare provider about any side effects you experience, as they may be able to adjust your treatment to alleviate your symptoms. 

Fluoxetine can also have serious side effects, which may require that you stop treatment or seek medical attention. If you experience suicidal thoughts or if you are considering harming yourself, speak to a healthcare professional immediately. 

Serious side effects of fluoxetine may include (UptoDate, n.d.):

• Allergic reactions like itching, skin rash, swelling of the face, throat, or lips, as well as swelling around your eyes, or hands, feet, or ankles

• Serotonin syndrome, or too much serotonin in the body, especially when combined with other medications that also affect serotonin levels

• Increased risk of bleeding in the digestive tract (Wee, 2017)

• Worsening of depression and suicidal thoughts/behaviors

This list is not exhaustive. It’s important to seek medical attention immediately if you have any serious symptoms while taking fluoxetine. 

Fluoxetine is a prescription drug that can only be taken by mouth. It is available in a liquid or pill form in 10, 20, or 40 mg doses. The typical starting dose of fluoxetine is 5–20 mg taken once daily, depending on the condition being treated. However, if you experience adverse reactions, your healthcare provider may consider lowering your dose. Dosing considerations are typically a function of the underlying condition, the effectiveness of the starting dose, and whether or not you experience side effects (UpToDate, n.d.). 

When combined with certain medications that also raise levels of serotonin, fluoxetine can cause a condition called serotonin syndrome. Symptoms of serotonin syndrome include confusion, agitation, fever, sweating, rapid or irregular heartbeat, shivering, severe muscle stiffness or twitching, hallucinations, loss of coordination, and digestive symptoms like nausea, vomiting, or diarrhea. Serotonin syndrome is dangerous and can even be deadly if left untreated. Medications that increase your risk of serotonin syndrome when combined with fluoxetine include (Simon, 2021): 

• MAOIs (like isocarboxazid and tranylcypromine) are medications used to treat depression and Parkinson’s disease, among other things. Do not use fluoxetine within 14 days of taking an MAOI. 

• Triptan medications such as sumatriptan (brand name Imitrex), almotriptan (brand name Axert), and naratriptan (brand name Amerge), among others. These medications are typically prescribed to treat migraine headaches.

• Other antidepressants like other SSRIs, serotonin-norepinephrine reuptake inhibitors (SNRIs) like duloxetine and venlafaxine, or tricyclic antidepressants like amitriptyline

• Other medications that may increase your risk for developing serotonin syndrome include linezolid, lithium, tramadol, buspirone, tryptophan, and the supplement St. John's Wort.

Because fluoxetine may increase a person’s chance of bleeding, be careful if you are taking other medications that increase your chance of bleeding, such as blood thinners (like warfarin and clopidogrel) and non-steroidal anti-inflammatory drugs (NSAIDs), including over-the-counter painkillers like ibuprofen and naproxen (UpToDate, n.d.). 

This list is not exhaustive. Be sure to tell your healthcare provider about any medications or supplements you are taking before starting a new treatment.

Fluoxetine carries a black box warning from the FDA: Rarely, people taking this medication, especially children, teenagers, and young adults up to 24, can develop suicidal thoughts or tendencies. If you are having suicidal thoughts, consult a healthcare professional immediately.

However, fluoxetine is considered to be a relatively safe medication in general. That said, there are some circumstances in which you should not use this medication (UptoDate, n.d.): 

• Do not take fluoxetine if you have an allergy to the medication or if you have ever had a reaction to any SSRI medications (UpToDate, n.d.).

• Fluoxetine should be used with caution in people over the age of 65 (Fick, 2019).

• Do not drink alcohol while taking fluoxetine, as this medication may increase the effects of alcohol on your body and make you feel drowsy. Also, there is some evidence that taking SSRIs may increase your risk for alcohol dependence (Brookwell, 2019).

• If you are pregnant, plan to become pregnant, or are breastfeeding, your healthcare provider will weigh the risks of taking this medication (UptoDate, n.d.). 

This list is not exhaustive. Consult with your healthcare provider regarding your medical history before starting treatment with fluoxetine. 

If you feel you need to discontinue fluoxetine, seek medical advice from your healthcare provider about how to slowly reduce your dose. Stopping abruptly may cause withdrawal symptoms, including dizziness, nausea, vomiting, headache, and irritability (Hirsch, 2020).

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Current version

June 22, 2021

Written by

Yael Cooperman, MD

Fact checked by

Chimene Richa, MD

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About the medical reviewers

Yael Cooperman, MD, Ro

Yael Cooperman is a physician and works as a Senior Manager, Medical Content & Education at Ro.

Chimene Richa, MD

Dr. Richa is a board-certified Ophthalmologist and medical writer for Ro.