Fluoxetine (Prozac)

What fluoxetine is

Fluoxetine is a medication also known by the brand name Prozac. It works mainly on serotonin, one of the chemical messengers nerve cells use to communicate, and medicines that work this way are often called SSRIs (selective serotonin reuptake inhibitors). It was one of the first medicines of its kind, and it is widely used and well studied.

What it treats

Your clinician might suggest fluoxetine for one of these, among other possible off-label uses:

• Depression
• Anxiety, including panic
• Obsessive-compulsive disorder
• Bulimia, an eating disorder
• Premenstrual dysphoric disorder (PMDD)

Whether it is a good fit depends on several factors, including your history and what you are working on. As with everything in psychiatry, that is decided case by case.

Who should not take fluoxetine. FDA labeling lists a few firm exclusions. Fluoxetine is not taken together with a group of medicines called MAOIs, because the combination can cause a dangerous reaction called serotonin syndrome; that group includes the antibiotic linezolid and methylene blue given into a vein. The waiting periods are not the same in both directions: fluoxetine is not started within 14 days of stopping an MAOI, and an MAOI is not started within 5 weeks of stopping fluoxetine. It is not taken with pimozide or thioridazine, and thioridazine is not started within 5 weeks of stopping fluoxetine. Make sure your clinician has your full medication list, including anything started recently or stopped within the last several weeks.

How it works

Honestly, no one knows with complete certainty how fluoxetine produces its benefit. What research suggests is that it helps keep more serotonin available in the spaces between nerve cells, and that this serotonin effect is central to how it helps. The biology is only part of the picture: how much a medication helps, and how it feels, is individual. Two people on the same medication can have very different experiences.

How to take it

There is no single right way to take fluoxetine; it depends on you and your clinician. Some people start low and increase slowly, others do not, and the timing and the form (capsule, tablet, liquid, or a weekly version) vary. The plan you and your clinician make together is the one to follow, not a number you read online. A few general points apply broadly:

• Take it the way you and your clinician agreed. It can be taken with or without food, and taking it with food can help if it bothers your stomach.
• Fluoxetine is long-acting: it and the active form it turns into leave the body slowly, over weeks rather than days. That tends to make a missed dose less disruptive and stopping gentler, but it also means dose changes take longer to fully settle, and the medication stays relevant to other medicines for a while after you stop. Your clinician factors this in, especially when switching to or from another medication.
• If you miss a dose, ask your clinician or pharmacist what to do rather than doubling up.
• Try not to stop on your own. Even though fluoxetine often eases off more gently than shorter-acting medicines, your clinician is the right person to plan any change if and when that makes sense.

What to expect

This varies from person to person. Some effects can show up before the benefit does: possible side effects, if they happen, often appear in the first days, while any lift in mood or easing of anxiety usually takes longer, commonly a few weeks and sometimes more. That early stretch is common and tends to ease. If nothing has shifted after a fair trial, that is useful information, not a dead end, and there are other options. As always, this is case by case.

Side effects

Not everyone gets side effects, and many that do happen ease over the first weeks. The lists below are possibilities, not certainties.

Possible more common side effects:

• Nausea, loose stools, or heartburn, especially early on
• Trouble sleeping, or feeling drowsy or tired
• Feeling nervous, anxious, or restless, especially early on
• Headache, dizziness, or a fine tremor
• Dry mouth, yawning, or sweating more than usual
• Changes in sex drive or sexual function

If any of these stick around or bother you, they are worth raising. Send a non-urgent message through the patient portal or bring it up at your next visit; often a small change helps. Restlessness or anxiety that is new, worsening, or severe is a reason to contact your clinician promptly rather than wait.

Less common, but concerning side effects that could require emergency care:

• Signs of an allergic reaction. Call 911 right away for swelling of the face, lips, tongue, or throat, or trouble breathing. Contact your clinician promptly about a new rash or hives.
• Serotonin syndrome: agitation, confusion, a racing heart, fever, heavy sweating, or stiff or twitching muscles, sometimes after another medication is added
• Unusual bleeding or bruising
• A seizure
• Feeling abnormally high, wired, or irritable, with racing thoughts and far less need for sleep than usual
• Eye pain or redness, or changes in vision such as blurred sight or seeing rings around lights
• Headache with weakness, confusion, trouble concentrating or remembering, or feeling unsteady
• Any new or worsening thoughts of harming yourself, whether or not you think the medication is behind it

For any of these, use the help options at the top of this page: call 911 or go to the nearest emergency department for a medical emergency or severe reaction, or call or text 988 for a mental health crisis.

If serotonin syndrome or a seizure is the concern, tell the clinicians treating you everything you take, including this medication. They may decide to stop it.

Pregnancy and breastfeeding

If you are pregnant, thinking about it, or breastfeeding, this is worth going through together rather than settling from a page. What follows is what the research currently shows. Tell your clinician before changing anything, and do not stop suddenly on your own.

In pregnancy

There is published information on more than 10,000 pregnancies exposed to fluoxetine in the first trimester, and no pattern of birth defects has been found. Most studies have not found a higher chance of birth defects. Some have reported more heart defects, and FDA labeling says those studies do not establish that fluoxetine was the cause. A small number of studies of miscarriage did not find an increase. For scale, about 3 in 100 babies are born with a birth defect, before any particular exposure is considered.

A few other findings come up often enough to name:

• Preterm birth and low birth weight. Some studies suggest a higher chance. Depression and anxiety are themselves associated with these outcomes, which makes the two hard to separate.
• A newborn lung condition. Some studies suggest that SSRIs taken in the second half of pregnancy might raise the chance of persistent pulmonary hypertension of the newborn, and others have not found this. It occurs in about 1 or 2 of every 1,000 births to begin with, and if SSRIs do raise it, the overall chance stays under 1 in 100.
• Bleeding after delivery. FDA labeling notes that SSRI use, particularly in the month before delivery, has been associated with a higher chance of postpartum hemorrhage. Worth making sure whoever delivers your baby knows what you are taking, well before the day.
• Later learning and behavior. A few studies that followed children from 16 months to 7 years of age found no differences. Most studies have not found a higher chance of ADHD, and most have not found a higher chance of autism once other factors are taken into account.

Symptoms in the newborn. Babies exposed late in pregnancy can have temporary symptoms soon after birth, and in most cases these are mild and go away within a few weeks. They can include irritability, jitteriness, trembling, more crying than usual, changes in sleep, tight or floppy muscle tone, a bluish tinge to the skin, trouble feeding, trouble holding a steady temperature, and trouble breathing. Not every exposed baby has them, and the chance may be higher when other psychiatric medications are taken as well. FDA labeling adds that some newborns have needed a longer hospital stay, help with breathing, or tube feeding. The useful thing you can do is make sure the delivery team knows about the medication ahead of time, so someone is watching for it.

While breastfeeding

More fluoxetine reaches breast milk than is the case for most other medicines of its kind, and LactMed, the National Institutes of Health lactation database, reports that its long-lasting breakdown product can be measured in the blood of most breastfed babies during the first 2 months. Colic, fussiness, and drowsiness have been reported in some babies. One study found slower weight gain and others did not. Infants followed for up to 5 years have not shown effects on development, and LactMed says that needing fluoxetine is not a reason to stop breastfeeding.

Reviewers do not all agree. LactMed reports that one safety scoring system rates fluoxetine as possible to use during breastfeeding while other reviewers do not recommend it. It also reports that most experts advise against switching medications for breastfeeding when fluoxetine was taken during the pregnancy or when other antidepressants have not worked, and that otherwise a medicine that passes into milk in smaller amounts may be preferred, especially with a newborn or a baby born early.

FDA labeling notes reports of breastfed babies with agitation, irritability, poor feeding, or poor weight gain. If you notice any of these, call the pediatrician.

Separately, some people taking an SSRI have a harder time getting breastfeeding established, and it is not clear how much of that is the medication and how much is the condition being treated. It is a good reason to line up lactation support early rather than to assume it will not work.

If you are a man taking fluoxetine

SSRIs can cause sexual side effects. Studies in a small number of men found that long-term use may affect sperm quality, although it stayed within the normal range and improved after the medication was stopped. A father's use of fluoxetine is considered unlikely to raise the chance of problems in a pregnancy.

Registries

There is a national registry that follows pregnancy outcomes for people taking psychiatric medications, and you can enroll yourself rather than waiting to be asked. It is listed in the references at the bottom of this page.

When to reach out, and where

For routine questions, side effects that can wait, or how things are going, send your clinician a message through the patient portal. These are part of your ongoing care and are answered in the normal course of a few business days, so they are best for things that are not urgent.

If something feels urgent, you do not need to wait for a reply. The help options at the top of this page are the fastest way to get care: 911 or the nearest emergency department for a medical emergency or severe reaction, or 988 any time for a mental health crisis or thoughts of self-harm.

Questions to ask your clinician

• What are you hoping fluoxetine will help with in my case?
• How and when will we know if it is working?
• What should I do if I notice side effects?
• How long should I give it before we reassess?
• Does its staying in the body a long time change anything for me, especially around other medications?
• How will we handle stopping it, if and when we get there?