Fluoxetine (Prozac)

What it treats

Fluoxetine is approved by the U.S. Food and Drug Administration to treat major depressive disorder in adults and children 8 and older, obsessive-compulsive disorder in adults and children 7 and older, bulimia nervosa, and panic disorder. It is also approved for premenstrual dysphoric disorder, where it is sold under the brand name Sarafem for that use. Combined with the antipsychotic olanzapine, it is approved for treatment-resistant depression and for bipolar depression.

Prescribers also use fluoxetine for other anxiety conditions. That is an off-label use, meaning a purpose the label doesn't formally list even though evidence and practice support it.

How it works

Fluoxetine is a selective serotonin reuptake inhibitor, usually shortened to SSRI. Nerve cells in the brain pass messages using chemical messengers, and serotonin is one of them. After a cell releases serotonin, it normally reabsorbs much of it, a process called reuptake. Fluoxetine slows that reabsorption, so more serotonin stays available between cells.

How that eases depression and anxiety isn't fully understood. The popular "chemical imbalance" explanation is an oversimplification, and it's worth being honest about that. What is clear is that the early change in serotonin signaling sets off slower adjustments in the brain over the following weeks. Those slower changes are thought to do the real work, which is part of why the medication takes time rather than working the day you start it.

What to expect

Common side effects

Most people get some side effects. The common ones include:

• Nausea and reduced appetite.
• Headache and dry mouth.
• Trouble sleeping.
• Nervousness or jitteriness.
• Drowsiness, for some people.
• Diarrhea.
• Increased sweating.
• Mild tremor.
• Sexual side effects.

Many of the early effects, including nausea and the jittery feeling, ease within one to two weeks as the body adjusts. Increased sweating and sexual side effects tend to last as long as the medication is taken rather than fading. If a side effect is severe, or it isn't improving after a few weeks, that's a conversation to have with the prescriber rather than a reason to stop on your own.

Serious side effects and warnings

Serious problems are uncommon, but a few are worth knowing.

Boxed warning. Like all antidepressants, fluoxetine carries an FDA boxed warning that it can increase suicidal thoughts and behaviors in children, teenagers, and young adults under 25, especially in the first weeks of treatment or after a dose change. This does not mean the medication harms most people. It means the early period deserves close attention, and that any worsening of mood, agitation, or new thoughts of self-harm should prompt contact with the prescriber promptly.

• Serotonin syndrome. A rare reaction caused by too much serotonin activity, most likely when fluoxetine is combined with other drugs that raise serotonin. Signs include agitation, a fast heartbeat, high body temperature, shivering, muscle twitching, and confusion. It is a medical emergency.
• Increased bleeding risk. Fluoxetine can make bleeding and bruising more likely, especially alongside NSAIDs such as ibuprofen or naproxen, aspirin, or blood thinners.
• Low sodium. A drop in blood sodium can happen, more often in older adults. Symptoms can include headache, confusion, and weakness.
• Mood switch in bipolar disorder. In people who have bipolar disorder, an antidepressant can sometimes trigger a manic or agitated state, which is one reason an accurate diagnosis matters.

One feature sets fluoxetine apart from other SSRIs. It has the longest half-life of the group, which means its active form lingers in the body for weeks after the last dose. This has two consequences. It makes fluoxetine gentler to stop, so discontinuation symptoms are usually milder and less common than with shorter-acting SSRIs. It also means the medication's effects and interactions persist for weeks after you stop taking it, which matters when switching to certain other drugs.

Sexual side effects

Fluoxetine, like other SSRIs, commonly affects sexual function, and it deserves a direct answer rather than a footnote. It can lower sex drive, delay orgasm or make it hard to reach, and cause arousal or erection difficulties.

These effects usually last as long as the medication is taken, rather than fading like nausea does. They are worth raising with a prescriber, because there are real options: lowering the dose, switching to a medication less likely to cause this, such as bupropion, or adding another medication to counter it. A small number of people report sexual side effects that continue after stopping the drug. This is uncommon and not well understood, but it is a real phenomenon and worth knowing about before you start.

Weight, appetite, and sleep

Fluoxetine is roughly weight-neutral, and some people lose a little appetite early on. It is one of the SSRIs less associated with weight gain.

Because fluoxetine tends to be activating, it more often disturbs sleep than causes drowsiness. For that reason it is usually taken in the morning. If it makes you sleepy instead, a prescriber may suggest a different timing.

Starting and dosing basics

This section is general background, not a dosing instruction for any individual. The right dose is a decision for a prescriber.

Fluoxetine comes as capsules, tablets, a liquid, and a once-weekly capsule. A common starting dose is 20 mg a day, and the usual range is 20 to 80 mg a day. The dose is adjusted gradually based on how a person responds and tolerates it. It can be taken with or without food.

Missed doses and interactions

If you miss a dose, the general guidance is to take it when you remember, unless it is almost time for the next dose. In that case, skip the missed dose and carry on. Don't take two doses to make up for one.

Several interactions matter. Fluoxetine must not be combined with MAOI antidepressants. Because of its long half-life, a long gap, around five weeks, is needed after stopping fluoxetine before an MAOI can be started. Combining it with other drugs that raise serotonin, such as triptans for migraine, tramadol, other antidepressants, or the supplement St. John's wort, increases the risk of serotonin syndrome. It should not be taken with pimozide or thioridazine. Fluoxetine can also change how the body handles some other medications, which can affect their levels.

Alcohol is not formally prohibited, but it isn't recommended. It can worsen side effects, disturb sleep, and work against the mood benefit you're taking the medication for. Give every prescriber and pharmacist a full list of your medications and supplements, including over-the-counter ones.

Stopping and tapering

Fluoxetine is not addictive in the usual sense of that word. It does not cause cravings or compulsive use. The body does adjust to it, but because of fluoxetine's long half-life, the active drug clears slowly on its own. That means discontinuation symptoms are usually milder and less frequent than with shorter-acting SSRIs.

Even so, stopping should still be done with a prescriber. Deciding to stop because you feel better is understandable, and sometimes it is the right call, but it is still worth doing with guidance so the timing and any switch to another medication are planned properly.

Pregnancy and breastfeeding

This is an area where individual circumstances matter and the decision belongs with a clinician. Untreated depression and anxiety carry their own risks during pregnancy. Fluoxetine passes into breast milk, in somewhat higher relative amounts than some other SSRIs, which is one factor a clinician weighs. None of that adds up to one answer that fits everyone. Anyone who is pregnant, planning a pregnancy, or breastfeeding should talk it through with their prescriber so the specific risks and benefits can be weighed for their situation.

Cost and generic availability

Fluoxetine has been available as a generic for many years and is inexpensive. The brand name Prozac and generic fluoxetine contain the same active medication and work the same way. Most insurance plans cover it, and for people paying out of pocket, generic fluoxetine is among the lower-cost antidepressants.

Common questions

How long until fluoxetine works? Some early effects on sleep and appetite can show within one to two weeks. The fuller effect on mood and anxiety usually takes four to six weeks, sometimes up to eight.

Is fluoxetine addictive? No, not in the usual sense. It doesn't cause cravings or compulsive use. The body does adapt to it, but because of its long half-life, stopping tends to be smoother than with other SSRIs.

Can I drink alcohol while taking it? It isn't banned, but it isn't recommended. Alcohol can worsen side effects and sleep and can undercut the benefit.

Will it change my personality? It shouldn't. When it works well, most people say they feel more like themselves, not numbed or different. If you feel flat or not yourself, tell your prescriber.

Should I take it in the morning or at night? Usually the morning, because fluoxetine tends to be activating and can disturb sleep if taken later in the day.

Questions to ask your prescriber

• What are we hoping this treats, and how will we know it's working?
• Which side effects should I expect early, and which ones should I call about?
• How long should I plan to take it?
• What's the plan if it doesn't help enough?
• If we decide to stop it later, how would we do that safely?

Sources

• U.S. Food and Drug Administration. Fluoxetine (Prozac) prescribing information.
• MedlinePlus, U.S. National Library of Medicine. Fluoxetine.
• National Institute of Mental Health. Mental health medications.
• American Psychiatric Association. Practice guideline for the treatment of patients with major depressive disorder.