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Can antibiotics cause weight gain in adults?

Antibiotics are medications used to treat bacterial infections. They work by killing bacteria or preventing them from multiplying. Some of the most commonly prescribed antibiotics include amoxicillin, azithromycin, cephalexin, ciprofloxacin, clindamycin, doxycycline, and metronidazole. While antibiotics are effective at treating infections, they can sometimes cause side effects like diarrhea, nausea, vomiting, and abdominal pain. Recently, there has been concern that antibiotics may also lead to weight gain in some people. In this article, we’ll explore the evidence on whether antibiotics can cause weight gain in adults.

Do antibiotics directly cause weight gain?

There is limited evidence that antibiotics directly cause significant weight gain. Most studies show minimal weight fluctuation during and shortly after taking a course of antibiotics. However, some antibiotics, especially certain tetracyclines and sulfonamides, have been associated with small amounts of weight gain. One study found that adults taking minocycline, a type of tetracycline antibiotic, gained on average 1.3 kg over a 6-month period. Weight gain was greatest in the first few months of use. Another study showed that patients taking sulfonamide antibiotics gained an average of 1.9 kg over a year of treatment. The amount of weight gain varied between different types of sulfonamides. Overall though, most studies show that antibiotics prescribed for common bacterial infections like respiratory, skin, and urinary tract infections do not directly lead to clinically significant weight gain. The average weight gain, if present at all, is modest – likely less than 2 kg over the course of treatment.

How might antibiotics indirectly affect weight?

Although antibiotics may not directly cause weight gain, there are some indirect ways that antibiotics could influence weight by altering the bacteria in the gut microbiome:

1. Change in calorie absorption from food

The gut microbiome helps digest and absorb nutrients from food. Different types of gut bacteria can affect how many calories are absorbed from carbohydrates and proteins. When antibiotics kill certain bacteria, the types and ratios of bacteria in the gut change, which affects calorie absorption. This could theoretically lead to slight weight fluctuations.

2. Change in energy harvest from food

The gut microbiome also helps extract energy from indigestible carbohydrates like fiber. When gut bacteria change, it can affect how much energy is harvested from certain foods. Some studies show that antibiotics may decrease the absorption of calories from fiber, possibly contributing to very modest weight loss. However, the overall energy change appears small.

3. Change in hormones related to hunger and satiety

The gut microbiome may also affect hunger and fullness by regulating hormones like ghrelin and leptin. When levels of these hormones are altered, it can increase or decrease appetite. Some early studies suggest antibiotics may influence ghrelin and leptin levels in ways that could theoretically impact weight, but more research is needed.

4. Change in fat storage

Some research indicates that the types of bacteria in the gut may impact how the body stores fat. When the microbiome is disrupted by antibiotics, it can potentially change fat regulation. However, there is little clear evidence that antibiotics directly lead to increased fat storage and significant weight gain.

Overall, changes to the gut microbiome from antibiotics could theoretically impact weight in subtle ways. However, clinical studies have not shown that antibiotics directly cause major weight changes. The microbiome tends to revert back to normal within weeks after antibiotic treatment stops.

Do some antibiotics cause more weight gain than others?

Most classes of antibiotics have not been linked with any significant weight gain. Tetracyclines and sulfonamides may have a higher chance of subtle weight gain compared to other antibiotic types, but even these classes do not appear to directly cause clinically concerning weight changes in most people.

Within antibiotic classes, some individual medications may have more metabolic effects than others. For example, minocycline has been associated with slightly more weight gain than other tetracyclines like doxycycline. However, there are few head-to-head studies comparing different antibiotics’ effects on weight. Overall, evidence does not clearly show that certain antibiotics consistently lead to substantially more weight gain than others across different patients.

Do long-term or repeated antibiotics cause more weight gain?

Longer-term use of antibiotics does appear more likely to cause modest weight gain compared to short-term use. With prolonged use of several months to years, certain antibiotics like minocycline and sulfonamides may lead to small amounts of weight gain.

One study found adults gained about 0.9 kg per year with long-term minocycline use. Other studies show that long-term sulfonamide antibiotics can lead to around 1 to 3 kg of weight gain over 1 to 3 years.

Frequent and repeated courses of antibiotics may also be more likely to disrupt the gut microbiome and affect weight compared to isolated short courses. Some evidence suggests repeat antibiotic exposure is associated with increased child and adult weight. However, there are few studies looking specifically at the effects of recurrent antibiotic courses on weight gain.

Overall, prolonged or frequent antibiotic use may cause subtle shifts in weight over time. But even long-term antibiotic therapy does not appear to directly lead to extreme, unhealthy weight changes in most people.

Do antibiotics cause more weight gain in children?

Some evidence suggests antibiotics may impact weight more in children compared to adults. Studies show that multiple antibiotic courses in the first 2 years of life are associated with slightly increased weight, BMI, and obesity risk later in childhood.

Possible reasons antibiotics may affect weight more in children include:

– Developing gut microbiomes may be more vulnerable to disruption.

– Microbiome changes can have longer-term effects when they occur early in life.

– The microbiome strongly regulates growth and development.

– Childhood weight gain patterns can persist into adulthood.

However, any weight changes in children after antibiotics tend to be quite small. One study found just a 4% higher BMI after exposure to antibiotics in infancy. Overall, antibiotics do not appear to be a major direct cause of excessive weight gain and obesity in children.

Can probiotics help prevent antibiotic-associated weight gain?

Some probiotic strains may help reduce weight gain after antibiotic use by restoring a healthy gut microbiome. Specific strains of Lactobacillus and Bifidobacterium have shown potential for mitigating weight gain in animal studies.

A few human studies also show hopeful results:

– In one study, women who took Lactobacillus rhamnosus GG after antibiotic treatment gained less weight over 6 months compared to those who took a placebo.

– Another study in children found that those who took a mix of Lactobacillus and Bifidobacterium had smaller increases in BMI after antibiotics compared to children who did not take probiotics.

However, more research is needed to confirm which probiotics are most effective for preventing antibiotic-associated weight changes. Probiotic supplements may not be necessary for most healthy adults taking a short course of antibiotics but could be beneficial for those on prolonged antibiotic therapy. Consult a doctor before taking any new dietary supplements.

Can antibiotics lead to long-term weight gain?

Evidence does not support antibiotics as a direct cause of major, long-term weight gain. Most studies show that any minor weight fluctuations after antibiotics are temporary. Within weeks to months of stopping antibiotics, the gut microbiome and weight typically return to normal.

However, antibiotics may indirectly impact weight by contributing to some of the root causes driving obesity, such as:

– Altering hormone levels related to fat storage and metabolism.

– Shifting digestive activity and calorie absorption.

– Increasing appetite and food intake behaviors.

– Promoting inflammation and insulin resistance.

These effects are complex, variable between individuals, and often reversible. While antibiotics may play a modest role, other factors like diet, inactivity, genetics, and the broader environment appear to drive most long-term weight changes.

Overall, antibiotic use does not directly cause lasting weight gain in most people. Any antibiotic-associated weight changes are usually small and unlikely to persist long after stopping treatment.

Tips to prevent weight gain on antibiotics

Here are some tips to help prevent weight gain when taking a course of antibiotics:

Take probiotics

Consume probiotic foods like yogurt, kefir, kimchi, sauerkraut, or kombucha or take a probiotic supplement to help maintain a healthy gut microbiome.

Eat prebiotic fiber

Prebiotic fibers like inulin, garlic, onions, leeks, asparagus, bananas, and oats can stimulate growth of beneficial gut bacteria.

Stay hydrated

Drink plenty of non-caloric fluids like water to avoid dehydration, which can sap energy and appetite.

Monitor diet

Be mindful of dietary changes that may increase calorie intake and lead to weight gain.

Exercise regularly

Engage in regular physical activity to help maintain a healthy body weight.

Get enough sleep

Adequate sleep supports a healthy metabolism and appetite regulation.

Weigh regularly

Track your weight to catch any significant fluctuations early.

Talk to your doctor

Discuss any concerns about weight gain with your healthcare provider.


In most cases, evidence suggests antibiotics do not directly cause significant, long-term weight gain. Subtle temporary increases of a few pounds may occur in some individuals, likely due to complex effects on gut bacteria. Longer antibiotic courses, frequent dosing, and childhood exposure may increase chances of minor weight fluctuations. However, antibiotic use alone is rarely a primary driver of major weight gain or obesity. Any antibiotic-associated weight changes tend to be small, reversible, and unlikely to persist over the long term after stopping treatment. Still, it is reasonable to be cautious of metabolic effects and take steps like probiotic supplementation when on long or repeated antibiotic therapy. As always, maintaining a balanced, nutrient-dense diet and active lifestyle are the foundations of healthy weight management.