Do antibiotics make you gain weight?

Our metabolism–or the rate at which our bodies use energy–exists in a delicate balance that can be thrown off, for some people, by a range of different things, among them perhaps antibiotics. If you’ve noticed weight gain while using antibiotics, know that you’re not alone. 

While the research on the subject is a little fuzzy, there have been a handful of studies that suggest long-term antibiotic usage may in fact contribute to weight gain. In this article, we’ll explore how antibiotics can affect the gut and what to do if you suspect that your antibiotic treatment has led to excess weight.

Do antibiotics make you gain weight? Put simply, maybe. Studies show that chronic antibiotic use can cause weight gain in adults and children, alike. Some research has shown that antibiotic use in utero and during early childhood can predispose people to extra weight gain as they grow. 

While research supports that there is a link between antibiotics and weight gain, experts don’t have a clear explanation for the phenomenon. One strong suggestion is through the way antibiotics interact with the gut microbiome, which is the population of bacteria that normally live in your digestive system. 

But how exactly do antibiotics impact bacteria in the gut?  Let’s get into it.

We all have a lot of bacteria in our bodies, also known as our microbiome. In fact, you have more “foreign” bacterial cells in your body than your own cells. Wild. And while most of that bacteria lives happily in your body and even helps you with things like digestion, there are also bad players that we can catch from other people or food, for example, that can make us sick.

Antibiotics are typically used to rid us of “bad” bacteria that make us sick, but in the process, they take out some of our good bacteria, too. That includes healthy cultures in our gut that support regular digestion and efficient metabolism––two important players in weight management and weight loss.

Studies have shown that broad-spectrum antibiotic use decreases gut flora diversity (the range of different types of bacteria in the gut) in the short term. And while the long-term impact is less clear, when our gut is compromised, so is its ability to perform its tasks––like those that support our metabolism.

To better understand how antibiotics impact the gut, let’s bulk up our understanding of the gut microbiome and why it is important for weight management. 

The gut microbiome and weight gain

A healthy microbiome plays a role in regulating fat storage. In a balanced gut, bacteria help support your metabolism. Disruption of the microbiome has also been linked to features of diabetes like insulin resistance and high blood sugar, as well as to high cholesterol. All of these factors are associated with metabolic syndrome which is a systemic condition that plays a role in weight gain.

Unrelated to antibiotic usage, research has also shown that people have overweight or obesity may have different microbiome compositions than people who are not. However, these researchers note that whether these findings are cause or coincidence is unclear, so more studies are needed to confirm their findings. But while it can be hard to tease out the exact relationship between gut bacteria and the number on the scale, it’s hard to deny that the connection exists.

Since it remains unclear exactly how or why some people might find themselves gaining weight on antibiotics, there’s no clear-cut way to prevent it, but there are some tips and tricks you can explore that might help. 

Don’t overuse or misuse antibiotics

First and foremost, don’t take antibiotics unless you have a prescription for them from a healthcare provider. If you have some pills left over from a previous bout of strep throat that you were supposed to finish but never did, another scratchy throat isn’t a reason to reach for the meds. No matter what you’re feeling, there’s a reason antibiotics are by prescription only, and it’s important to remember that they won’t help you if you don’t have a bacterial infection.

Another thing to remember is that there are different types of antibiotics, so the meds you were prescribed for your throat way back when might not be the right thing if you find yourself with a UTI. 

So if you’re feeling unwell, make an appointment with your primary care provider who can determine if you might have something that needs antibiotics and then prescribe the right medication for the right amount of time. 

Talk to your doctor about weight gain concerns 

Speak with your healthcare provider about any concerns you have about weight gain. They can give you their expert advice, tailored to your situation. If you have a history of overweight or obesity, they may suggest considering a broader lifestyle change or even weight loss medications if appropriate.  

Eat a healthy diet (don’t skip the fiber!)

Eating nutritious foods can support your gut health, which can support your metabolism and has other weight-management properties. Foods that are good sources of fiber, like sweet potatoes or beans, and foods that have active cultures, like yogurt, can also support gut health. 

Move your body

Physical exercise can support multiple throughways in your body that fight weight gain, on and off antibiotics. Plus, studies show that exercise can alter the gut biome in ways that can benefit your overall health. So taking a long walk or dancing in your kitchen can be good for you in more ways than one. 

Take a probiotic

Taking a probiotic alongside antibiotics can encourage a healthy gut during the course of your treatment. (But keep in mind that if you regularly take a probiotic, it may be less effective while you are on antibiotics.) Research shows that probiotics may also fight against other unwanted side effects of antibiotic treatment, like diarrhea. 

If you find you’ve gained weight after a round of antibiotics, it doesn’t mean that added weight is permanent. Speak with your healthcare provider about whether your antibiotics might be contributing to your weight fluctuations and what options you have.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

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Current version

February 15, 2024

Written by

Claire Wolters

Fact checked by

Yael Cooperman, MD

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About the medical reviewer

Yael Cooperman, MD, Ro

Yael Cooperman is a physician and works as a Senior Manager, Medical Content & Education at Ro.