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Weight changes are common side effects of selective serotonin reuptake inhibitors (SSRIs) like Prozac (fluoxetine). However, whether Prozac causes weight gain or weight loss varies by the individual. In other words, the answer to the question “Does Prozac cause fluctuations in body mass?” isn’t straightforward.

Does It Cause Fluctuations in Body Mass?

It can, although the changes are usually indirect side effects of Prozac.

Weight Loss

Some patients report modest, short-term weight loss (0.4 kg/0.88 lbs) when taking this medication. Experts believe that fluoxetine itself does not cause this drop in weight. Rather, they believe that this small weight loss is due to alleviating symptoms of the condition Prozac is treating. For example, a patient treating depression may notice a slight drop in weight if fluoxetine helps stabilize their appetite.

Weight Gain

Other studies suggest that prolonged Prozac use may lead to an increase in weight. For example, one 2-year study found that fluoxetine non-smokers gained an average of 4.6 lbs while smokers gained an average of 6.7 lbs. These weight changes were similar to other SSRIs, exempting sertraline users who experienced significantly greater weight gain (roughly 10.5 lbs over 2 years).

Final Thoughts

Some people experience fluctuations in weight when taking Prozac. Data suggests that these changes are merely side effects of improving the conditions that this drug treats, though, rather than being a direct effect of taking Prozac itself. Patients concerned about drastic weight loss or weight gain associated with fluoxetine should contact their medical provider.

Disclaimer: this article does not constitute or replace medical advice. If you have an emergency or a serious medical question, please contact a medical professional or call 911 immediately. To see our full medical disclaimer, visit our Terms of Use page.


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