Prozac (Fluoxetine) is considered one of the most popular antidepressant medications of all time. It was developed by the pharmaceutical company Eli Lilly and approved for the treatment of major depression in 1987. Many people have had great success taking Prozac to manage depression, OCD, panic attacks, as well as eating disorders.

The drug functions as an SSRI (selective-serotonin reuptake inhibitor) which increases extracellular levels of the neurotransmitter serotonin. By increasing serotonin levels in the brain, many individuals find that it works great to help manage depression. Despite the fact that it can really help some people cope with psychiatric conditions, those taking it often complain of weight gain.

Prozac and Weight Gain

When people are prescribed an SSRI medication like Prozac, they are mostly focused on recovering from a mood disorder like depression – they aren’t concerned with potential side effects. In some cases a doctor won’t even inform patients of certain side effects with the thought that if a patient knows of side effects, they may discontinue a treatment that they need. In any event, many people end up noticing throughout the course of their Prozac treatment that they’ve packed on some extra pounds.

How Prozac Causes Weight Gain (Theories)

There are many theories as to how Prozac and other SSRI medications may cause weight gain. Some speculate that Prozac may slow the metabolism, while others believe that it increases craving for certain sugary foods like refined carbohydrates. Others may simply experience an appetite change while on the drug. In any regard, below is the laundry list of possibilities that may contribute to gaining weight on this medication.

  • Appetite changes: If you had a poor appetite before treatment, and all of a sudden you have a strong appetite after taking the drug, it’s obvious that you’re going to gain weight. Many people with depression don’t eat enough food while they’re depressed, and when they start treatment, their appetite returns and they start to eat. If you notice that your appetite changes, realize that this is a normal side effect from the drug.
  • Carbohydrate cravings: There is a relationship between serotonin and craving carbohydrates. Some believe that Prozac and other SSRI medications alter serotonin, which in turn causes some people to experience carbohydrate cravings. If you all of a sudden develop a sweet tooth and are eating more pastas while on this drug, it may be a result of this factor.
  • Drug interactions: It is important to consider that the drug may be interacting with another medication or drug that you are taking. The interaction between the two (or more) drugs may be causing you to gain weight. It is also important to consider the influence of the other drug on your weight before blaming Prozac.
  • Fat storage: Taking this medication may be increasing your body’s natural propensity to store fat. If your body starts packing on more fat than you were prior to taking the drug, it may be due to this particular factor.
  • Hormonal changes: Another idea is that Prozac creates hormonal changes in the body over the course of treatment. When hormone levels become altered, it can lead to increased hunger, fat storage, etc. Due to the fact that SSRI-induced hormonal changes has not been extensively studied, this is difficult to confirm.
  • Improved taste: Some have noticed that when they take a certain medication, foods taste significantly better. It could be argued that neurotransmitter alterations in the brain may also alter perception of taste. Those who now experience an improvement in taste are more likely to eat more frequently.
  • Less depressed: When people are less depressed from taking Prozac, they may be more likely to eat. When depressed, a person typically isolates themselves, eats less, and may not care about their personal health. Upon becoming less depressed, they may seek out food and better manage their hunger.
  • Motivational changes: Taking an antidepressant may result in a decrease in overall motivation. In other words, a person may feel so much better with their life, that they may see no need to exercise. This is a phenomenon that occurs in some people and could be called antidepressant-induced laziness. Those taking Prozac may become lazy as a result of the drug’s effects.
  • Side effects: The side effects of Prozac can include things like fatigue, tiredness, and grogginess. Not everyone will feel tired after taking the medication, but some people certainly do. Side effects like increased tiredness and decreased energy may lead a person to sleep longer and rest more instead of getting proper exercise.
  • Slow metabolism: Your metabolism may be the root cause of the entire Prozac-induced weight gain. Some speculate that regardless of whether you eat the same foods and portions during treatment as you did pre-treatment, you are still going to gain weight. This leads many to believe that the cause of this weight gain despite no change in foods or portions is a result of a slowed metabolism.
  • Social dining: Those who feel less depressed while taking an antidepressant often start to go out more often with friends. Engaging in social activities often results in “social eating” which generally isn’t healthy food. Social eating often results in going to fast-food restaurants and/or large portions of food.

Note: It is possible to experience a combined influence from many of the possibilities listed above.  In other words, a specific theory such as slow metabolism may not be the sole culprit for causing the weight gain.  It could be a compounding effect as a result of slowed metabolism, plus feeling lethargic, plus social dining.

Factors that influence weight gain on Prozac

It is important to recognize that there are other factors that will influence weight gain that a person experiences while taking Prozac. Most notably, the dosage, your lifestyle, how long you’ve taken the drug, as well as if you are on any other medications are often responsible for your weight gain.

1. Dosage

How much Prozac do you currently take? Those taking a high dose are significantly more likely to gain weight in comparison to those taking a low dose. When you take a higher dose, there is increased likelihood for side effects as well as physiological alterations. Essentially at higher doses, you are giving the drug more influence over your brain as well as body.

If you gained a little bit of weight at a low dose, but double the dose, you are also likely doubling the weight gain. This is why it is highly important to always stay on the minimal effective dose when taking this drug.

2. Individual factors

There are many individual factors that may play influence weight gain while on Prozac. These include things like: environmental stress, baseline metabolism, physiology, genetics, hormones, dietary intake, exercise habits, and your overall lifestyle. Many people assume that the drug is always responsible for the weight that they gain throughout treatment and fail to realize that it could be a variety of other factors as well. If you start craving carbs and end up eating a poor diet, this factor alone could be responsible for your weight gain.

3. Time span

The time span over which you have been taking Prozac also will influence the amount of weight you gain. Taking the drug over a short term period like several weeks often doesn’t give the effect of the drug enough time to “build up.” When you take the drug for moths, the drug will have elicited its effects on your brain and entire physiology.

If you stay on it for an even longer term, you may gain more weight due to the fact that it may alter your physiological functioning to a more significant extent. Additionally when many people take Prozac for a long term, they end up doubling the dosage due to tolerance at a lower dose. The increases in dosing along with long-term effects on your functioning make it easier to gain weight.

4. Other medications

Before assuming that the Prozac is directly responsible for your weight gain, also consider other medications that you’re taking. In some cases it may be the other medication wreaking havoc on your weight more than the Prozac. In other cases it may be that both your Prozac and other medication are each causing some reaction that leads to weight gain.

Yet in other individuals, both drugs could be working in synergy to compound the amount of weight that you gain. Always be sure to look up the potential side effects of other medications to determine whether they are weight-neutral or associated with weight gain.

How much weight will you gain from Prozac?

There’s no telling exactly how much weight you will gain from taking Prozac. Some people taking high dosages may gain quite a bit. Those who didn’t eat much prior to treatment who notice that their appetite increases may also gain a significant amount of weight. On average, most people that gain weight from Prozac and other SSRIs tend to gain 10 lbs. throughout the course of treatment. Fortunately, most people don’t view 10 lbs. as a significant amount and this negligible weight gain won’t bother them as long as the drug is working well for their depression.  Other uncontrolled studies have suggested it is possible to gain nearly 21 lbs. after 6-12 months of treatment.

  • Source: http://www.ncbi.nlm.nih.gov/pubmed/24898363
  • Source: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC181155/

Will everyone gain weight while taking Prozac?

No. In fact, some people will actually lose weight for a variety of reasons while on this medication. Some people taking Prozac may actually lose interest in eating and/or become less hungry than usual. Others may not notice any weight change and for them the antidepressant will be considered “weight neutral.” It is estimated that nearly 1/4 people taking a particular SSRI drug like Prozac report weight gain. This means that for 75% of people, weight gain isn’t really much of a problem.

Prozac: Cost-Benefit Analysis

It is important to always conduct a “cost-benefit” analysis of any medication that you’re taking – including Prozac. If you gained a significant amount of weight, think about whether the drug is working well enough to justify this gain. For some individuals, the drug may not be working well and they may be gaining weight. In this case, a person may be better off going through Prozac withdrawal and/or switching to another medication.

Not everyone will have a great reaction to this drug, so finding a medication that you’re compatible with can be a challenge. If this medication causes you to gain some weight, but you feel significantly better than before you started taking it, it should be an easy decision to stay on it. Don’t let minor short term weight gain cloud your judgment into thinking that you’d function better without the drug if it has been working great.

Did you gain weight while taking Prozac?

Keep in mind that many people will gain no weight from taking this particular drug. For those that ended up gaining weight while taking Prozac, feel free to share how much weight you gained in the comments section below. It may help some to know the dosage that you were taking, how long you were taking it, and any other factors that you can think of that may have lead to an increase in weight. If you have any other thoughts regarding weight gain as a result of taking Prozac, be sure to mention them.