Initially, you may lose a small amount of weight, about 1 kg (2.2 lb) on average, when you first start taking Prozac, but over the long-term you may gain that weight back or even add on extra weight up to 6 kgs (13.2 lbs). Studies suggest this may be due to a recovery of your appetite after your symptoms subside. Weight changes may vary between patients, but in general Prozac is not usually associated with serious weight gain (over 7%).

In Prozac studies, weight loss has been reported in about 2% of patients (2 out of every 100), but patient self-reports of weight gain have been much higher, up to 37% in one survey.

Selective serotonin reuptake inhibitors (SSRIs) available in the U.S. include:

• Celexa (citalopram)
• Lexapro (escitalopram)
• Prozac (fluoxetine)
• Fluvoxamine (generic only)
• Paxil (paroxetine)
• Zoloft (sertraline)

Of all of the SSRIs, Prozac (fluoxetine) appears to be associated with the least chance for weight gain and Paxil (paroxetine) with the greatest chance. Paroxetine has been shown to lead to up to a 3.6% weight gain in about 6% of patients in one study. Other antidepressants, such the tricyclic antidepressants (TCAs) or mirtazapine can be associated with significant weight gain.

Why do weight changes occur with Prozac?

Weight gain

It is not fully clear if longer-term weight gain with selective serotonin reuptake inhibitors (SSRIs) is due to a drug side effect, the recovery of appetite after symptom recovery, or a mix of both. Short-term Prozac treatment for up to 3 months usually leads to little weight change, but most patients need medication for longer periods. Other reasons for weight gain with SSRIs may include genetic differences and increased carbohydrate cravings.

Weight loss

Weight loss during early treatment with Prozac may also be due to stomach (gastrointestinal) side effects. All SSRIs can cause some degree of short-term nausea and digestive tract discomfort when starting therapy or increasing the dose.

Over time, these common side effects tend to improve for most patients, but can include:

• loss of appetite (4% to 17% of patients)
• diarrhea (8% to 18% of patients)
• nausea (12% to 29% of patients)

Patients with diabetes may have trouble with low blood sugar levels while taking Prozac, but blood sugar levels may increase when treatment is stopped. Your healthcare provider may need to adjust your dose of diabetes medications.

Other common side effects with SSRIs include sexual dysfunction, insomnia, headache, dry mouth, sedation, sweating, tremor, agitation and orthostatic hypotension (low blood pressure, dizziness when standing). Serious side effects like suicidal thoughts, bleeding, abnormal heart rhythms can also occur with SSRIs.

Studies: Weight Changes with Prozac

In studies looking at weight changes with Prozac (fluoxetine), weight loss has ranged from about 0.2% of body weight to a gain of about 1%.

Study 1

In a one-year randomized study, patients whose depressive symptoms had subsided after 12 weeks of treatment with fluoxetine 20 mg/day received either fluoxetine or a placebo (an inactive treatment) for 38 weeks. Weight changes were assessed during the initial 12 weeks of treatment and after 14, 26, and 38 weeks (for a total of 50 weeks).

• During the initial 4 weeks of treatment, an average weight decrease of 0.4 kg (0.9 lb) was observed for all patients. After 50 weeks, weight increases were similar between the Prozac and placebo-treated groups. An average weight gain of 3 kg (6.6 lb) was observed over the entire 50-week period in the fluoxetine group, compared to 3.2 kg (7 lb) for the placebo group.
• Weight gain was associated with a poor appetite at the beginning of treatment and an improved appetite after depressive symptoms cleared. Weight gain was not associated with the patient’s body weight at the start of the study.
• No patients stopped Prozac treatment due to weight gain.

Study 2

In another study in 284 patients with major depressive disorder, long-term weight changes with Prozac were assessed when compared to paroxetine (Paxil) or sertraline (Zoloft), other commonly used SSRIs. Patients were evaluated for a total of 26 to 32 weeks.

• Researchers looked at the average change in weight in each group and the number of patients with at least a 7% increase in weight from the start of the study.
• Patients treated with paroxetine (Paxil) had a significant increase in weight, and those with a 7% or greater weight increase were highest in this group. Patients receiving fluoxetine (Prozac) or sertraline (Zoloft) had a modest but nonsignificant weight increase.

Bottom Line

• Prozac is associated with a low amount of weight gain over the long- term compared to many SSRIs. Early in treatment, during the first few weeks it can cause a slight weight loss, but usually only about a pound or so.
• It appears weight loss may be due to adverse stomach side effects like nausea or loss of appetite in the short-term, while weight gain may occur over the longer-term due to improved appetite and social functioning with improved mood.
• If you are concerned about weight gain or loss with any antidepressant treatment, speak with your healthcare provider. There are many options to treat depression, some that have fewer side effects like weight gain. Do not stop treatment on your own without speaking to your healthcare provider.

This is not all the information you need to know about Prozac (fluoxetine) for safe and effective use and does not take the place of your doctor’s directions. Review the full product information and discuss this information and any questions you have with your doctor or other health care provider.