Diarrhea From Antibiotics: What Causes It, How Long It Lasts

Do antibiotics cause diarrhea? Yes, commonly. Antibiotic-associated diarrhea (AAD), defined as three or more loose stools a day after starting an antibiotic, affects roughly 5% to 35% of people who take them, depending on the drug and the person^[1][2]. Most cases are mild and settle once the antibiotic stops^[3].

A smaller share is caused by Clostridioides difficile (C. diff), a germ that can inflame the colon. Estimates put it behind 10% to 20% of cases, and under a third in a more recent review^[3][1].

This page covers why antibiotic diarrhea happens, which drugs carry the most risk, how long it lasts, what to do, and how to spot the signs of C. difficile infection. For diarrhea from all causes, see our diarrhea guide.

Get medical help promptly if your diarrhea comes with blood, pus or black, tarry stools, a high fever, severe belly pain, or signs of dehydration such as dizziness and very little urine^[4]. During or after antibiotics, these can signal C. diff colitis, which needs a stool test and specific treatment^[5].

How do antibiotics cause diarrhea?

Antibiotic-induced diarrhea happens in three main ways: they thin out the gut bacteria that help digest food, some act directly on the gut, and the gaps they leave let harmful microbes overgrow^[1]. Most mild cases come from the first two^[3].

Fewer gut bacteria, more water in the colon

Gut bacteria ferment carbohydrates you do not absorb, such as fiber and resistant starch, into short-chain fatty acids like butyrate^[1]. When antibiotics cut those bacteria down, unfermented carbohydrate stays in the colon and holds water there, which causes osmotic diarrhea^[1]. Lower short-chain fatty acid production also deprives the colon lining of an efficient energy source^[1].

Bile acids change too. Fewer bacteria convert primary bile acids into secondary bile acids, and the leftover chenodeoxycholic acid can pull fluid into the gut (secretory diarrhea)^[1].

Direct effects on the gut

Some antibiotics change gut movement or irritate the lining regardless of the microbiome^[3]. Erythromycin, a macrolide, stimulates motilin receptors, which speeds stomach emptying and shortens transit time^[1]. Amoxicillin-clavulanate may increase gut contractions at night^[1]. This kind of diarrhea usually starts within 48 to 72 hours and tends to be mild and short-lived^[1].

Harmful microbes take over

A healthy microbiome resists newcomers by using up nutrients and space and making antimicrobial compounds, a property called colonization resistance^[1]. Once antibiotics open gaps, opportunists can grow^[6]:

• Clostridioides difficile: the most common identified cause, through toxins A and B^[1].
• Klebsiella oxytoca: linked to bloody, mostly right-sided colitis, typically after penicillins^[1].
• Clostridium perfringens and Staphylococcus aureus: less common, mainly in older or hospitalized patients^[3][1].

Which antibiotics cause the worst diarrhea?

Nearly any antibiotic can cause diarrhea, but the risk varies by drug class^[2]. Amoxicillin-clavulanate stands out for plain diarrhea, and clindamycin, fluoroquinolones and several cephalosporins for C. diff.

Absolute risk stays low for most people: among 36.6 million US adults given outpatient antibiotics, 0.03% developed community-acquired C. diff within 90 days^[9]. In older adults with other risk factors, nitrofurantoin was also linked to C. diff (odds ratio 3.05)^[9].

When does antibiotic diarrhea start, and how long does it last?

Antibiotic diarrhea can start within a few hours of the first dose or up to 8 weeks after starting the course^[1]. Mild cases usually last only while you take the drug and clear once it stops^[3].

Is diarrhea after finishing antibiotics normal?

It can happen, but post-antibiotic diarrhea deserves more attention than diarrhea during the course.

People are up to 10 times more likely to get C. diff while taking an antibiotic and in the month after finishing, and longer courses may double that risk^[5]. The CDC advises contacting a healthcare professional if diarrhea develops on or after antibiotics, since not every case is C. diff and only a stool test can tell^[5].

How long does it take for bowels to recover after antibiotics?

Mild diarrhea usually clears once the course ends^[3], but the gut microbiome takes longer.

In 12 healthy men given a 4-day course of three broad-spectrum antibiotics, gut bacteria returned to near-baseline within about 1.5 months^[10]. Nine common species were still undetectable in most of them after 6 months, and Bifidobacterium species and butyrate producers were among the early losses^[10]. The CDC notes that antibiotic effects on the gut microbiome can last several months^[5].

Persistent or chronic diarrhea after antibiotics is a different matter: C. diff infection can cause chronic diarrhea, so it needs checking^[4]. Our gut restoration protocol covers steps people take after a course of antibiotics.

What are the symptoms of antibiotic-associated colitis?

Colitis from C. diff causes diarrhea plus fever, belly tenderness or pain, nausea and loss of appetite^[5]. You cannot tell it from mild antibiotic diarrhea for sure without a stool test, but the pattern gives clues^[5]. Blood, fever, belly pain and very frequent watery stools point toward colitis rather than simple antibiotic diarrhea^[6].

How do you stop antibiotic diarrhea?

For mild diarrhea, the steps are fluids, sensible eating and a call to the prescriber, not stopping the drug on your own. Mild cases usually resolve when the antibiotic course ends or the clinician changes it^[3].

1. Keep taking the antibiotic and call the prescriber. A healthcare professional may stop it or switch to a different one if that is safe for your infection^[5].
2. Replace fluids and salts. Water, broths, sports drinks and oral rehydration solutions all help. Older adults and people with diabetes or kidney disease should check with a doctor before using rehydration solutions^[11].
3. Be careful with antidiarrheal medicines. Loperamide and bismuth are usually safe for acute diarrhea, but doctors generally advise against them with bloody stools or fever, and for infants and children^[11]. Ask first if you are on antibiotics.
4. Watch the clock. See a doctor if diarrhea gets worse or lasts more than 2 days^[11][4].

What should you eat, and which foods should you avoid?

Eat your normal diet once your appetite returns, even if stools are still loose^[11]. While on antibiotics, cut back on foods high in poorly absorbed carbohydrates, because the bacteria that normally ferment them are depleted^[3]. Sugar alcohols such as sorbitol, mannitol and xylitol, found in sugar-free sweets and some liquid medicines, can add to diarrhea^[4]. MedlinePlus suggests asking your provider about probiotic supplements or yogurt, continued for a few days after the course^[6].

Can probiotics lower the risk?

Meta-analyses suggest some probiotics taken alongside antibiotics may lower the risk, though the evidence is mixed in quality. Our full guide to probiotics for diarrhea covers strains and doses.

• A 2012 analysis of 63 trials (11,811 people) found a 42% lower relative risk (relative risk 0.58), with high variation between trials^[12].
• A 2021 analysis of 42 trials in 11,305 adults found a 37% reduction, with no clear benefit where baseline risk was low^[13].
• In children, a Cochrane review of 33 trials found diarrhea in 8% with probiotics vs. 19% without; doses of at least 5 billion CFU a day worked better^[14].
• For C. diff diarrhea, another Cochrane review found 1.5% vs. 4.0%, though many trials had a high or unclear risk of bias^[15].

Not everyone agrees. NIDDK notes that many doctors and professional societies do not think the evidence supports probiotics for preventing diarrhea, and advises asking a doctor first^[11]. The yeast Saccharomyces boulardii is one of the options studied for antibiotic diarrhea^[3]; our Florastor review looks at its trials, and our guide to when to take probiotics covers timing around antibiotic doses.

Who is most likely to get diarrhea from antibiotics?

Risk depends on the drug, your health and your exposure to germs^[2]. For C. diff specifically, the CDC lists these risk factors^[5]:

• age 65 or older;
• a recent stay in a hospital or nursing home;
• a weakened immune system, for example from cancer treatment or transplant drugs;
• a previous C. diff infection;
• a longer antibiotic course.

Children are affected too: about 19% of children on antibiotics in the control groups of pediatric trials developed diarrhea^[14]. NIDDK advises people over 65, pregnant people, those on antibiotics and anyone with a weakened immune system to keep in touch with their doctor when they have diarrhea^[4].

When to see a doctor

Contact a doctor right away if you or someone you care for has diarrhea with^[4]:

• stools that are black and tarry or contain red blood or pus;
• severe pain in the belly or rectum;
• high fever, or six or more loose stools a day;
• diarrhea that lasts more than 2 days in an adult, or more than 1 day in a child;
• signs of dehydration: extreme thirst, dry mouth, little or dark urine, dizziness, or no wet diapers for 3 hours in infants;
• frequent vomiting or a change in mental state, such as irritability or lack of energy.

Also call if diarrhea starts during or after a course of antibiotics and does not settle, especially if you have had C. diff before^[5].

This article is for general educational purposes and is not medical advice. It cannot tell you whether your diarrhea is caused by C. diff, which needs a stool test. Do not stop or change a prescribed antibiotic without talking to the prescriber, and talk to a qualified healthcare professional before using probiotics or antidiarrheal medicines, particularly if you are pregnant, immunocompromised, taking prescription medication, or managing a diagnosed condition.