Does Prozac Cause Weight Gain? Short- and Long-Term Evidence

The answer depends on the kind of evidence. In U.S. placebo-controlled depression studies, the Prozac label reports decreased appetite and weight loss more often with Prozac than with placebo. Those reports are not a rate of weight gain. Prozac prescribing information and Medication Guide on DailyMed, revised August 2023

Some people in longer studies gained weight. Those studies do not establish how much of that gain came from fluoxetine, the active ingredient in Prozac, rather than from recovery from depression, the passage of time or other changes in a person's life, and they cannot predict what will happen to you.

This guide is for anyone who takes Prozac or generic fluoxetine, or is discussing it with a prescriber, and wants to understand a change in weight or appetite. Majesta Health does not offer Prozac or psychiatric prescribing, and this article does not suggest taking fluoxetine to lose weight or changing depression or anxiety treatment through a weight-loss service.

Early weeks and longer treatment: the evidence at a glance

These are group results from different kinds of studies. None of them predicts one person's weight.

What the Prozac label says about weight and appetite

Prozac is approved in the United States to treat major depressive disorder, obsessive-compulsive disorder, panic disorder and the binge-eating and vomiting behaviors of moderate to severe bulimia nervosa. Used together with olanzapine, it is also approved for the acute treatment of depressive episodes of bipolar I disorder and for treatment-resistant depression. It is not approved for weight loss.

Of the depression studies above, the label adds that people only rarely stopped Prozac because of decreased appetite or weight loss. The label and its Medication Guide also report the following.

• Obsessive-compulsive disorder studies: decreased appetite was reported more often than with placebo.
• Bulimia nervosa studies: decreased appetite was reported more often than with placebo. In one 16-week study, the Prozac group lost 0.45 kg (about 1 lb) on average, while the placebo group gained 0.16 kg. This was one condition at one studied dose, so it does not describe everyone who takes Prozac.
• Children and teens: after 19 weeks in one study, those taking fluoxetine gained on average 1.1 kg less weight and 1.1 cm less height than those taking placebo. The label says height and weight should be monitored periodically in children and teens taking fluoxetine.
• The Medication Guide: its list of possible serious side effects includes changes in appetite or weight, without saying in which direction, and its list of common side effects includes loss of appetite.

The label also warns that significant weight loss, especially in underweight people with depression or bulimia, may be an undesirable result of treatment. In the same section, after the bulimia study, it says weight change should be monitored during therapy. So tell your prescriber about unplanned weight loss or a lasting loss of appetite too, not only about gain.

None of this rules out weight gain for one person. The label gives no rate of weight gain for adults.

What longer studies found

Three longer studies add information, and each has limits.

A randomized comparison of three antidepressants. In a double-blind study published in 2000, 284 patients with major depressive disorder were randomly assigned to fluoxetine, sertraline or paroxetine for 26 to 32 weeks. The fluoxetine group had a modest weight decrease that was not statistically significant. There was no placebo group, and the results cover only those who finished: 44 of the 92 assigned to fluoxetine. Fava et al., Journal of Clinical Psychiatry, 2000

A study of people whose depression improved. In a 1999 study, patients with depression took fluoxetine for 12 weeks. Those whose depression remitted were randomly assigned to keep taking it or switch to placebo for up to 38 more weeks. Among those who finished all 50 weeks, average weight gain during the later phase was similar in the two groups. Gain was linked with poor appetite at the start and with appetite improving after recovery, and the authors concluded that it was most likely related to recovery from depression. Michelson et al., American Journal of Psychiatry, 1999

That explanation fits the results, but the study does not prove it.

• Everyone took fluoxetine first, including the people later switched to placebo, and the later comparison included only people whose depression remitted.
• The later result covers only those who finished. We read the study's published abstract, which does not say how many finished or what share of people gained weight.
• A link between returning appetite and weight gain does not show which caused which, and it may not apply to you.
• The lead author's listed affiliation is Lilly Research Laboratories, part of the company that makes Prozac.

A large study of health records. A 2024 study used records from eight U.S. health systems for 183,118 adults who started one of eight common antidepressants. It compared each medicine with sertraline (Zoloft), not with taking no antidepressant. Petimar et al., Annals of Internal Medicine, 2024

• Starting fluoxetine versus starting sertraline. Estimated average weight change was 0.07 kg (about 0.15 lb) less at 6 months, 0.06 kg less at 12 months and 0.20 kg (about 0.44 lb) less at 24 months. At each point, the 95% confidence interval, the range that shows how uncertain an estimate is, included no difference. The estimated chance of gaining at least 5% of starting weight was also close to sertraline's, with no clear difference at any point.
• Not a measure of Prozac's own effect. The authors report that average weight went up with every medicine they studied. With no group taking no antidepressant, they treated differences between medicines as the least biased result. So these figures do not show how much weight Prozac adds or prevents, or that it is weight-neutral or better than Zoloft.
• Few people kept taking it. By the study's prescription-based definition, 19% of people who started fluoxetine were still taking it at 12 months and 5% at 24 months, and 15% had a weight recorded in month 24.
• People who kept taking it. A secondary analysis tried to estimate what happens when people start a medicine and keep taking it. Compared with sertraline, it estimated a lower chance of gaining at least 5% of starting weight on fluoxetine at 6 months, no clear difference at 12 months and a higher chance at 24 months. That later estimate rests on the few people still taking fluoxetine and on statistical weighting for those who stopped or had no weight recorded, so it raises a question about long continuous use rather than answering it.

The study was observational, had no consistent dose information, and recorded prescriptions written, not whether medicines were filled or taken. These are comparisons between groups taking two medicines, so they cannot tell you how your own weight will change.

Our guide to sertraline (Zoloft) and weight covers the comparison medicine, and a separate guide covers escitalopram (Lexapro) and weight. These averages are not a reason to switch medicines on your own. A decision about treatment also depends on how well it is working for you, and it belongs in a conversation with your prescriber.

Why a weight change can have more than one cause

Timing alone cannot prove that Prozac caused a change, and it cannot rule it out. These are areas your prescriber may want to look at with you.

• Depression itself. The National Institute of Mental Health lists changes in appetite or unplanned weight changes among symptoms of depression, and says problems with sleep, appetite and concentration often improve before mood lifts on an antidepressant. NIMH: Depression, revised 2024
• Other medicines. NIDDK lists some medicines used for depression, epilepsy or psychotic disorders, corticosteroids, and some diabetes, blood pressure and allergy medicines among those that may contribute to weight gain. It does not name Prozac. NIDDK: factors affecting weight and health If you also take gabapentin, our guide to gabapentin and weight covers it separately.
• Prozac combined with olanzapine. The combination has its own label, which warns about weight gain and says weight should be monitored regularly. The evidence in this article does not cover that combination, so ask about it directly. Olanzapine and fluoxetine prescribing information on DailyMed, revised March 2026
• Health and life changes. MedlinePlus lists an underactive thyroid, polycystic ovary syndrome, menopause, pregnancy and quitting smoking among possible reasons for unintended weight gain. MedlinePlus: unintentional weight gain
• Fluid. MedlinePlus also warns that rapid weight gain may be a sign of dangerous fluid retention, and it says to contact your provider about swollen feet and shortness of breath with weight gain. Contact a clinician promptly if your weight rises quickly or you notice new swelling. Call 911 if breathing trouble starts suddenly or is severe. MedlinePlus: breathing difficulty Our guide to water weight and when swelling needs care explains more.

If you feel stuck, our calm review of why the scale may not be moving walks through these areas.

Before you change anything

Do not stop taking Prozac or change how you take it on your own because of a weight change. The Medication Guide says not to stop Prozac without first talking to your healthcare provider, because stopping it too quickly may cause serious symptoms, such as anxiety, mood changes, dizziness, nausea and electric shock-like sensations. The Prozac Medication Guide on DailyMed has the full list. It also says not to start or stop any medicine while taking Prozac without talking to your healthcare provider first. That includes a weight-loss medicine.

Fluoxetine also leaves the body slowly. The label says a dose change is not fully reflected in the blood for several weeks, and that active drug can remain in the body for weeks after the last dose. That is one more reason not to test a change on your own. If you and your prescriber decide a change makes sense, they can plan it with you and tell you what to watch for.

When to get help right away

Prozac has a boxed warning: antidepressants increased the risk of suicidal thoughts and behavior in children, teens and young adults in short-term studies, and people of all ages who start an antidepressant should be closely monitored for worsening and for new suicidal thoughts and behaviors.

The Medication Guide says to call your healthcare provider right away, or 911 in an emergency, about symptoms such as thoughts about suicide or dying, a suicide attempt, new or worse depression or anxiety, panic attacks, agitation, acting on dangerous impulses, or other unusual changes in mood or behavior, especially if they are new, worse or worry you.

In the U.S., if you or someone you know is in danger or having a medical emergency, call 911 or go to the nearest emergency room. You can call or text 988 for crisis support.

A checklist for your next prescriber visit

Copy what helps into a notebook or a phone note. It is a way to organize what you have noticed, not a schedule you have to keep.

What to write down

• Your medicine: Prozac or generic fluoxetine, why it was prescribed, and anything it is combined with.
• Timing: the date you started, the dates of any dose changes, and when you first noticed the change.
• What changed: your weight if you know it and how fast it changed, appetite, cravings, how clothes fit, and any swelling.
• Other medicines: every prescription and over-the-counter medicine, vitamin, mineral and supplement, including any you started or stopped recently. MedlinePlus advises keeping this list and bringing it each time you visit a doctor. MedlinePlus: fluoxetine
• Other changes: sleep, activity, illness, stress, pregnancy, menopause, quitting smoking, or changes in your mood or anxiety.
• How treatment is going: whether Prozac is helping with what it was prescribed for.

If weighing yourself feels distressing, you do not need to do it for this checklist. Note other changes, such as appetite or how clothes fit, and tell your clinician, who can suggest another way to follow your progress.

What to ask

1. Could this change be related to Prozac, to changes in my symptoms, or to something else in my health or routine?
2. Should anything be checked, such as blood tests or my other medicines?
3. How well is Prozac working for what it was prescribed for, and how should we weigh that against this change?
4. Is there anything you would like me to track before our next visit, and can I do that without weighing myself?
5. If a change in treatment is ever considered, how would we plan it, and what should I watch for?
6. Who should I contact between visits if my mood gets worse or a side effect worries me?

This article is for informational purposes only and does not constitute medical advice, diagnosis or treatment. It does not recommend starting, stopping or changing any medicine. Sources checked November 9, 2026. The Prozac label on DailyMed shows an update date of January 8, 2026 and text revised August 2023.