Antibiotic-Associated Diarrhea

Why antibiotics cause diarrhea

Your large bowel contains trillions of bacteria that digest fiber, make short-chain fatty acids and keep harmful bacteria in check. Antibiotics kill many of these helpful bacteria along with the infection-causing ones. This can cause diarrhea in two ways:

• Simple (non-infectious) AAD: fewer bacteria are available to digest carbohydrates and process bile acids, so water is drawn into the bowel. Some antibiotics, such as erythromycin and amoxicillin-clavulanate, also directly speed up gut movement.
• Infectious AAD: with less competition, harmful organisms overgrow. C. difficile is the most important. Others include Klebsiella oxytoca (a cause of bloody diarrhea), some Staphylococcus aureus strains and Candida.

Simple AAD vs C. diff infection

Risk factors

• Which antibiotic: clindamycin, broad-spectrum penicillins (especially amoxicillin-clavulanate), cephalosporins and fluoroquinolones carry the highest risk
• Longer courses and multiple antibiotics
• Age over 65
• Hospital or nursing home stays, where C. diff spores are more common
• Acid-reducing medicines (proton pump inhibitors)
• Weakened immune system, chemotherapy, inflammatory bowel disease
• A previous C. diff infection

C. diff infection

Clostridioides difficile forms hardy spores that survive on surfaces for months and are not killed by alcohol hand gel. After antibiotics, swallowed spores can germinate in the colon and release toxins that damage the bowel lining.

Severity ranges widely:

• mild to moderate watery diarrhea with cramps
• pseudomembranous colitis: severe inflammation with yellow plaques on the colon lining, fever and dehydration
• fulminant colitis and toxic megacolon: a dangerously dilated colon, which can perforate and is life-threatening

About 1 in 6 people who have C. diff get it again within 8 weeks. See C. diff colitis.

Diagnosis

Mild diarrhea during a short antibiotic course often needs no tests. Doctors test for C. diff when someone has 3 or more unexplained unformed stools in 24 hours after antibiotic exposure. Tests include:

• Stool tests for C. diff toxins and genes (only on liquid stool; testing formed stool or people without diarrhea is not recommended)
• Blood tests: white cell count, kidney function and albumin to judge severity
• CT scan for severe pain or suspected complications
• Colonoscopy occasionally, to look for pseudomembranes

Treatment

Simple AAD

• Keep drinking: water, broths and oral rehydration solutions. Older adults and children dehydrate faster.
• Eat normally as tolerated. Bland, starchy foods may be easier.
• Talk to your prescriber if diarrhea is bothersome. They may stop the antibiotic or switch to one less likely to cause diarrhea. Do not stop it on your own.
• Avoid anti-diarrheal medicines such as loperamide until C. diff has been excluded, because slowing the bowel can worsen C. diff colitis.

C. diff infection

• Stop the triggering antibiotic if possible.
• Specific antibiotics: fidaxomicin or oral vancomycin are recommended first-line treatments in adults. Metronidazole is now used mainly when these are unavailable for mild cases.
• Recurrent infection: repeat or tapered courses, fidaxomicin, bezlotoxumab (a monoclonal antibody), or fecal microbiota transplantation (FMT), including approved microbiome products, which are highly effective for multiple recurrences.
• Severe colitis: hospital care, IV fluids, and sometimes surgery to remove the colon.

Prevention

• Take antibiotics only when needed, and never for colds or flu.
• Probiotics: some strains, such as Saccharomyces boulardii and certain Lactobacillus combinations, may reduce the risk of simple AAD. Evidence for preventing C. diff is mixed, and guideline recommendations vary. People who are severely ill or immunocompromised should ask a doctor first.
• Wash hands with soap and water after using the toilet and before eating, especially around someone with C. diff. Alcohol gel does not kill C. diff spores.
• Clean bathrooms with bleach-based products if someone at home has C. diff.
• Tell doctors if you have had C. diff before, so antibiotic choices can be adjusted.

Frequently asked questions

How long does diarrhea from antibiotics last?

Simple antibiotic-associated diarrhea usually stops within a few days to 2 weeks after finishing the antibiotic. Diarrhea that is frequent, watery, getting worse, or starts weeks after the course may be C. diff and should be tested.

Should I stop my antibiotic if I get diarrhea?

Not without talking to your doctor or pharmacist. Mild diarrhea is common and often tolerable. Your prescriber may switch you to a different antibiotic or stop it if the infection is already treated. Severe diarrhea or fever needs prompt assessment.

Do probiotics help antibiotic-associated diarrhea?

Some probiotics, especially Saccharomyces boulardii and certain Lactobacillus strains, may lower the chance of developing diarrhea while on antibiotics. Their effect on C. diff is less certain. They are generally safe for healthy people but not for those who are very ill or immunocompromised.

How do I know if it is C. diff?

C. diff typically causes 3 or more watery, foul-smelling stools a day, cramping, fever and loss of appetite, during or up to a few months after antibiotics. Only a stool test can confirm it, so see a doctor if you have these symptoms.