Antibiotics cause diarrhea by killing beneficial gut bacteria alongside the harmful ones, which disrupts digestion and fluid absorption in your colon. This side effect, called antibiotic-associated diarrhea, affects 5% to 35% of people who take antibiotics, depending on the drug, the dose, and individual health factors.
How Gut Bacteria Normally Protect You
Your colon relies on trillions of bacteria to function properly. One of their most important jobs is breaking down the roughly 70 grams of undigested carbohydrate that reaches your large intestine every day. Colonic bacteria, especially certain anaerobes, ferment these carbohydrates and produce short-chain fatty acids. Your colon absorbs these fatty acids easily, which pulls water along with them and keeps your stool at a normal consistency.
One of these fatty acids, butyrate, also serves as the primary energy source for the cells lining your lower colon. So the bacteria aren’t just helping with digestion. They’re literally feeding the tissue that absorbs water from your stool.
These same resident bacteria also act as a physical barrier. They occupy space and compete for nutrients, which prevents harmful organisms from gaining a foothold. Researchers call this “colonization resistance,” and it’s one of the main reasons most people carry small numbers of potentially dangerous microbes without ever getting sick.
What Antibiotics Do to This System
Broad-spectrum antibiotics can’t distinguish between the bacteria causing your infection and the ones keeping your gut running. When you take them, the diversity of your gut microbiome drops. Vulnerable species, particularly beneficial anaerobes like Bifidobacteria, get wiped out or dramatically reduced. Even when a species survives in terms of overall numbers, antibiotics can cause rapid shifts in its genetic composition, meaning the surviving bacteria may function differently than before.
This disruption triggers diarrhea through two connected pathways. First, with fewer bacteria to ferment carbohydrates, undigested sugars and fiber accumulate in your colon. These molecules draw water into the intestinal space through osmosis, producing loose, watery stool. Second, the drop in butyrate production starves colonic cells of their preferred energy source, impairing the colon’s ability to absorb water and electrolytes normally. The combination of excess water being pulled in and less water being absorbed is what creates that urgent, frequent diarrhea.
Which Antibiotics Carry the Highest Risk
Nearly all antibiotics can cause diarrhea, but some classes are more likely to trigger it. The antibiotics most often involved are cephalosporins, fluoroquinolones, penicillins, clindamycin, carbapenems, and macrolides. Broader-spectrum drugs, which target a wider range of bacteria, tend to cause more disruption than narrow-spectrum ones because they eliminate a greater portion of your gut’s microbial community.
The CDC specifically flags fluoroquinolones, carbapenems, and third- and fourth-generation cephalosporins as the highest risk antibiotics for serious gut complications, particularly C. diff infection. This is one reason why antibiotic stewardship programs push doctors to prescribe the narrowest effective antibiotic for the shortest effective duration.
When C. Diff Becomes the Problem
Most antibiotic-associated diarrhea is mild and resolves on its own. But in some cases, the disruption opens the door for a dangerous bacterium called Clostridioides difficile. Under normal conditions, your resident bacteria keep C. diff in check through colonization resistance. When antibiotics wipe out those protective communities, C. diff can colonize the open territory, multiply rapidly, and produce toxins that directly damage the cells lining your colon.
C. diff infection is a fundamentally different problem from ordinary antibiotic diarrhea. The toxins cause inflammation and can lead to a condition called pseudomembranous colitis, where the colon becomes severely inflamed. Symptoms that point toward C. diff rather than garden-variety antibiotic diarrhea include watery diarrhea five or more times per day, fever, severe abdominal cramping, and bloody stool. If you develop any of these, particularly blood in your stool or more than five episodes of diarrhea daily lasting beyond one to two days, that warrants prompt medical attention.
Typical Timeline
Diarrhea can start at any point during your antibiotic course, sometimes within the first few days. For most people, symptoms are mild, with a few extra loose stools per day, and resolve shortly after finishing the medication. Your gut microbiome begins recovering once antibiotic exposure stops, though full restoration of diversity can take weeks to months depending on the drug and your starting gut health.
C. diff is the exception. It can appear during antibiotic treatment or up to several weeks after you’ve finished. This delayed onset catches some people off guard because they assume the risk is over once they stop taking the pills.
Managing Symptoms During Treatment
Staying hydrated is the most important thing you can do. Frequent diarrhea depletes both water and electrolytes, so drinking water along with fluids that contain electrolytes (broths, sports drinks, or electrolyte solutions) helps prevent dehydration. This matters especially for older adults and young children, who dehydrate faster.
Probiotics are widely marketed for this purpose, and the idea makes intuitive sense: replace the bacteria being killed off. The evidence, however, is mixed. Some studies have suggested that a yeast-based probiotic called Saccharomyces boulardii may reduce the relative risk of antibiotic-associated diarrhea by roughly 60%, but researchers have noted the quality of that data is still weak. Taking probiotics is unlikely to cause harm for most people, but those with weakened immune systems should avoid probiotic supplements.
You don’t need to stop your antibiotic just because you have loose stools. Mild diarrhea is a common trade-off of treatment. However, if your symptoms become severe, with high-volume watery diarrhea, blood, fever, or significant cramping, contact whoever prescribed the antibiotic. They may need to switch you to a different drug or test for C. diff.
Why Some People Are More Affected
The same antibiotic can cause significant diarrhea in one person and none in another. Several factors influence your risk. A longer course of antibiotics exposes your gut to more disruption. Taking multiple antibiotics simultaneously compounds the damage. People with lower baseline gut diversity, whether from previous antibiotic courses, chronic illness, or age, have less microbial resilience to begin with.
Certain medications may also compound the problem. Proton pump inhibitors, commonly taken for acid reflux, are hypothesized to increase the risk of C. diff infection, which is why some hospitals evaluate whether patients truly need them during antibiotic treatment. Age is another factor: older adults in healthcare settings face the highest rates of C. diff, partly because of more frequent antibiotic exposure and partly because aging itself reduces gut microbial diversity.