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Fluoxetine side effects and how to avoid them

Fluoxetine side effects include weight loss, decreased libido, and diarrhea
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By | Oct. 11, 2022 Medically reviewed by Kristi C. Torres, Pharm.D.

Common fluoxetine side effects | Serious side effects | Weight loss | Decreased libido | Diarrhea | Side effects timeline | Contraindications | Warnings | Interactions | How to avoid side effects | How to treat side effects

Fluoxetine—better known by its brand-name version, Prozac—is a generic prescription antidepressant prescribed for major depressive disorder (MDD), obsessive-compulsive disorder (OCD), panic disorder, and the eating disorder bulimia nervosa. In addition, the Food and Drug Administration (FDA) has approved fluoxetine in combination with the antipsychotic medication olanzapine for treatment-resistant depression or depressive episodes associated with bipolar disorder. As a selective serotonin reuptake inhibitor (SSRI), fluoxetine helps reset chemicals in parts of the brain responsible for mood control. Like all drugs that affect the nervous system, fluoxetine commonly produces a variety of side effects, but many can be managed or prevented.

Common side effects of fluoxetine

By far the most common side effect of fluoxetine is insomnia. In clinical trials, anywhere from 16% to 33% of test subjects reported problems sleeping. The incidence varied, however, on the condition being treated and the size of the dose. The other most frequently reported side effects were nausea, headache, fatigue, and loss of appetite. The most common side effects of fluoxetine are:

  • Trouble sleeping
  • Nausea
  • Headache
  • Weakness or fatigue
  • Diarrhea
  • Loss of appetite
  • Drowsiness
  • Anxiety
  • Tremor
  • Dry mouth
  • Yawning
  • Decreased libido
  • Indigestion
  • Sore throat
  • Dizziness
  • Excessive sweating
  • Erectile dysfunction
  • Ejaculation dysfunction
  • Hives
  • Skin rash
  • Nasal congestion
  • Abnormal dreams
  • Flushing
  • Constipation
  • Flatulence
  • Vomiting
  • Itching
  • Weight loss
  • Nosebleed
  • Vision problems
  • Weight loss

Serious side effects of fluoxetine

The most serious side effects of fluoxetine include:

  • Drug-induced movement problems (extrapyramidal effects)
  • Suicidal thoughts and behaviors
  • Abnormal heartbeats
  • Heart problems
  • Seizures
  • Bleeding
  • Serotonin syndrome
  • Mania or hypomania
  • Low sodium
  • Low blood sugar
  • Angle-closure glaucoma
  • Scarring of the lung (pulmonary fibrosis)
  • Prolonged or painful erections (priapism)
  • Serotonin syndrome
  • Bleeding
  • Severe allergic reactions

Weight loss

Weight loss is not very common when taking fluoxetine. In the initial clinical trials, about 2% of patients taking fluoxetine reported weight loss as opposed to 1% who were taking a placebo. However, 10% of trial participants had problems with decreased appetite. For this reason, fluoxetine is sometimes used off-label for weight loss, but the results are not the best. As with any drug, if your weight goes down unexpectedly, inform the prescribing healthcare provider.

Decreased libido

Fluoxetine, like other SSRIs, can decrease both sexual interest and function. In clinical trials, 4% of participating patients taking fluoxetine lost some interest in sex, significantly greater than the 1% of patients given a placebo. In addition, some men experience ejaculation problems, and some women have reported difficulty reaching orgasm while taking fluoxetine. Fluoxetine and other SSRIs do reduce genital sensitivity. Get medical advice if one’s sex life is affected by fluoxetine. The good news is that genital sensitivity, sexual desire, and sexual function should return to normal when SSRIs are stopped.

Diarrhea

In clinical studies, 11% of participants had at least one diarrhea episode, but 7% of people taking an inert placebo did, too. So fluoxetine can definitely cause diarrhea. Talk to the prescriber. However, if diarrhea gets pretty bad or doesn’t get better, get medical attention. 

Fluoxetine side effects in females

Fluoxetine may be more effective as an antidepressant in women of reproductive age than either older women or men. However, there is no agreement that men and women may have gender-specific differences in side effects. All that can be said is that fluoxetine affects people differently, whether for good or bad. 

How soon do fluoxetine side effects start?

Some possible side effects such as insomnia, nausea, drowsiness, dry mouth, and diarrhea could be experienced after the first dose or within the first few days of treatment. These adverse effects may improve over time. Other side effects may take a few weeks of treatment before becoming a problem. These include sexual problems, fatigue, loss of appetite, and many of the worst side effects such as suicidality and heart problems.

How long do fluoxetine side effects last?

The good news is that most side effects such as insomnia, diarrhea, and drowsiness usually get better after the drug is discontinued. It may take a few days since it can take almost two weeks to clear just half a dose of fluoxetine from the body. Some side effects, such as genital numbing, sexual problems, sleeplessness, and suicidal thoughts, may linger for two or more weeks after the drug is stopped. 

What are the long-term side effects of fluoxetine?

Fluoxetine is often used safely for months or years. There are specific side effects due to the long-term use of the drug. However, some severe side effects such as heart problems or glaucoma may have lifelong consequences even after the drug has been stopped.

Fluoxetine contraindications

The only people who should never take fluoxetine are those who are allergic to the drug or other SSRIs. 

Children, adolescents, pregnant women, breastfeeding women, and older adults can take fluoxetine. However, healthcare providers are cautious about giving it to kids and teens because of the risk of worsening depression and suicidal thoughts. They are also cautious about women taking fluoxetine during the third trimester of pregnancy because of possible risks to the baby when it’s born. The same applies to breastfeeding. Because fluoxetine is present in breast milk, it is not recommended for nursing women.

Fluoxetine warnings

Though only contraindicated for people allergic to SSRIs, fluoxetine may still cause problems in certain populations. 

Black box warning

Fluoxetine—like all other SSRIs—contains a black box warning about the risk of worsening depression and suicidality. The overall incidence is between 0.1% and 1%, but the risk is very high in children, adolescents, and young adults. 

Cautions

Some people have health conditions that may cause problems when they’re put on fluoxetine, including: 

  • Heart problems: Anyone with congestive heart failure, electrolyte imbalance, a recent heart attack, or a family history of heart rhythm abnormalities may need to be checked regularly to make sure fluoxetine is not causing heart problems. The use of certain other drugs at the same time as fluoxetine may increase this risk.
  • Poor metabolizers: Some people don’t break down fluoxetine very well, putting them at risk for heart problems, too. Doses may need to be reduced. 
  • Bleeding disorders: Fluoxetine can interfere with blood clotting, so it’s risky when given to people who already have bleeding problems. This risk increases in patients who are on other drugs which affect their ability to form clots such as aspirin, NSAIDs, and warfarin.
  • Diabetes: Fluoxetine can cause low blood sugar.
  • History of seizures: Fluoxetine should be used cautiously in patients with a history of seizure disorder..
  • Older people: There’s a greater risk of side effects and low sodium.
  • Dehydration: Low sodium is also a potentially serious risk in people who are low on fluids.
  • Bipolar disorder: It is believed that a depressive episode may be the initial presentation in bipolar disorder. It is thought that initiating treatment with an antidepressant alone may increase the chance of a mixed or manic episode. Patients being seen for a depressive episode should be adequately screened for bipolar disorder. This may lead your physician to make a different choice for initial therapy. 
  • Uncorrected narrow angles: People with narrow angles in their eyes can get angle closure glaucoma, a rapidly progressing glaucoma that can end in blindness in just a few days. Fluoxetine raises that risk.

Abuse and dependence

Fluoxetine has not been associated with abuse, drug seeking, or withdrawal. The sudden discontinuation of fluoxetine can precipitate unpleasant withdrawal symptoms, so talk to the prescribing healthcare provider about the best way to stop the medicine if it’s causing problems.

Overdose

Taking too much fluoxetine can be fatal, so a suspected overdose will need emergency medical treatment. If unsure, watch for symptoms of an overdose such as:

  • Seizures
  • Sleepiness
  • Racing heartbeat
  • Nausea and vomiting

Fluoxetine interactions

More than a few drugs can cause problems with fluoxetine. There are many of them, so think of them in terms of the problems they may cause.

Heart problems

Fluoxetine can cause the heart to beat all wrong, so the following drugs can never be combined with fluoxetine:

Though not prohibited, heart rhythm drugs combined with fluoxetine can also raise the risk of abnormal heartbeats.

Serotonin syndrome

A potentially fatal side effect, the risk of serotonin syndrome is increased when combined with the following drugs that also raise serotonin in the brain:

Bleeding

Because fluoxetine interferes with blood clotting, taking fluoxetine along with drugs that also block blood clotting creates an increased risk for bleeding:

Other problem drugs

Fluoxetine is broken down by a certain liver enzyme. Some drugs tie up that enzyme, so fluoxetine levels in the body could then build up, raising the risk of side effects. The list of these drugs is long, so make sure the prescribing healthcare provider or the pharmacist is aware of all prescription drugs, over-the-counter medications, and even supplements that are being taken. They’ll be able to spot the problem drugs. 

How to avoid fluoxetine side effects

Side effects of fluoxetine are not entirely avoidable, but a few tips can help keep them at arm’s length:

1. Tell the prescriber about all medical conditions

The first place to start is a complete medical history. In this way, the prescribing healthcare professional can take measures to prevent some of the most serious side effects. The most important are:

  • Heart problems
  • Stroke
  • History of seizures
  • Drug abuse
  • Suicidal thoughts
  • Bipolar disorder or a history of mania
  • Bleeding problems
  • Diabetes
  • High blood pressure
  • Bleeding problems
  • Uncorrected narrow angles or a history of narrow-angle glaucoma
  • Electroconvulsive therapy (ECT) treatments
  • Liver problems
  • Kidney or urination problems
  • Sexual problems
  • Pregnancy or pregnancy plans
  • Breastfeeding or breastfeeding plans

2. Tell the prescriber about any drugs being taken

Step two is to tell the healthcare provider about all drugs being taken. Make a complete list and keep it on hand at all times. For fluoxetine, the most important are:

  • MAO inhibitors
  • Antidepressant medications, anxiety drugs, antipsychotics, bipolar medications, or similar mental health drugs
  • Amphetamines
  • Migraine medications called triptans
  • Opioids like tramadol or fentanyl
  • Blood thinners 
  • Supplements or herbal remedies like St. John’s wort or tryptophan.

3. Follow all instructions

Do what the healthcare provider, pharmacist, and prescribing label say to do. Don’t change the dose. A missed dose can be taken as soon as it’s remembered, but not if it’s almost time for the next dose. Just take the next dose as scheduled, but don’t add to the dose. 

4. Take the dose in the morning

Trouble sleeping is the most common problem associated with fluoxetine. To avoid this problem, take the daily dose early in the morning.

5. Do not drink alcohol

Drowsiness and dizziness are also major problems. Drinking will only make them worse.

6. Avoid some pain relievers

When combined with fluoxetine, NSAIDs can cause abdominal pain and bleeding, as well as bruising and bleeding elsewhere. Put those drugs on hold for a while and use acetaminophen instead.

7. Eat a healthy diet

Weight changes could be a problem, so start right away with a healthy diet.

How to treat side effects of fluoxetine

There’s a good chance that anyone on fluoxetine will have some problems. Fortunately, most will not be serious, but they could be. Here’s what to do just in case.

Drowsiness and dizziness

While not much can be done to help to make dizziness or drowsiness better, a few measures can keep one safe from falls and injuries. Stand up slowly, move carefully, and remove any obstacles that can cause injuries if fallen on. 

Weight changes

An unwanted increase or decrease in weight, though sometimes welcome, should be told to the prescriber. Consider talking with a dietitian or nutritionist. Chances are that the best treatment is a change in diet.

Low sodium

Low sodium could rapidly become a serious problem, so call the prescribing healthcare provider when noticing symptoms such as:

  • Headache
  • Weakness
  • Coordination problems
  • Confusion

The standard way to treat mild cases of low sodium is to increase salt intake. However, seizures are a sign of serious sodium depletion, so emergency medical treatment is required.

Suicidal thoughts and behaviors

Any person exhibiting suicidal thoughts and behaviors requires immediate medical help. That person’s safety is of paramount and immediate concern, so call the prescriber, a healthcare professional, or mental health professional at any significant sign of suicidality including:

  • Suicide attempts or plans
  • Talking or thinking about suicide or death
  • New or sudden changes in mood or behavior
  • New or worsening depression or anxiety
  • Aggressive, hostile, or violent behavior
  • Reckless or risky behaviors
  • Unusual behaviors
  • Agitation, restlessness, irritability, or anger
  • Panic attacks
  • Extreme increase in activity or talking
  • Trouble sleeping

Serotonin syndrome

Drugs like fluoxetine may cause serotonin levels in the brain to get too high and essentially put the brain into overdrive. Serotonin syndrome is usually uncomfortable at the very least and can be life-threatening at the very worst. Get emergency medical help at any sign of serotonin syndrome including:

  • Fast heartbeat
  • Changes in blood pressure
  • Sweating
  • Fever
  • Rigid muscles
  • Dizziness
  • Flushing
  • Agitation
  • Hallucinations
  • Tremor
  • Seizures

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