Why Do Antibiotics Cause Diarrhea, and When Could It Be C. diff?

Antibiotics often cause loose stools because they disturb the balance of bacteria in your gut, and most of the time this is mild and settles after the course ends. Sometimes the same disruption lets Clostridioides difficile (C. diff) overgrow, which can cause watery diarrhea, belly pain and fever, and can become serious. This article explains why antibiotics upset digestion, how ordinary diarrhea differs from C. diff, and which symptoms need urgent care.

Why antibiotics loosen your stools

Your large intestine is home to trillions of bacteria, collectively called the gut microbiome. They help break down food, produce some vitamins and keep harmful organisms in check. Antibiotics are designed to kill the bacteria causing an infection, but most of them cannot tell the difference between the bacteria making you sick and many of the helpful ones in your gut.

Several factors probably contribute to the diarrhea that follows. When helpful bacteria are reduced, less carbohydrate may be fermented normally, and more water can stay in the stool. The gut lining and its movement can also be affected. Some antibiotics, such as erythromycin and related drugs, can speed up gut movement directly. The exact mechanism differs from drug to drug and is not fully understood.

The CDC notes that antibiotics can cause side effects including diarrhea, and that these side effects are one reason to use antibiotics only when they are needed. If you are wondering whether you needed one at all, see our article on why antibiotics do not help a cold or the flu.

What ordinary antibiotic-associated diarrhea looks like

Ordinary antibiotic-associated diarrhea is usually mild. Typical features include:

• Loose or soft stools, often a few more than usual
• Mild cramping or gas
• Onset during the course of antibiotics or shortly after starting
• Improvement once the antibiotic is finished, or sometimes sooner
• No high fever, no blood in the stool and no severe belly pain

Almost any antibiotic can cause it. It is often discussed with broad-spectrum medicines such as amoxicillin-clavulanate, cephalosporins and clindamycin, but it can occur with others. Our guide to amoxicillin covers that drug's side effects in more detail.

What C. diff is and why antibiotics raise the risk

Clostridioides difficile is a bacterium that many healthy people carry without symptoms. It can also be picked up from surfaces and hands, especially in hospitals and long-term care facilities, because it forms hardy spores. According to the CDC, C. diff infection can cause diarrhea and colitis (inflammation of the colon), and taking antibiotics is a major risk factor.

The idea is straightforward. When antibiotics reduce the normal gut bacteria, C. diff may have room to multiply. It releases toxins that damage the lining of the colon. This can happen while you are taking the antibiotic, and it can also appear weeks afterward. The CDC describes the risk as extending for some time after antibiotics are stopped, so a new bout of diarrhea after you finish a course still counts.

Who is more likely to develop it

Risk is higher in people who:

• Are 65 or older
• Have recently stayed in a hospital or nursing home
• Have taken antibiotics, especially for a longer time or more than one
• Have a weakened immune system or inflammatory bowel disease
• Have had C. diff before, since it can come back
• Take certain stomach-acid-reducing medicines, which have been associated with higher risk in some studies

Younger, otherwise healthy people living in the community can also get it, so a lower-risk profile does not exclude it.

Ordinary diarrhea versus possible C. diff

Symptoms overlap, which is why clinicians combine your history, an examination and stool testing rather than relying on one feature. The comparison below is a rough guide, not a way to diagnose yourself.

The CDC describes the main C. diff symptoms as watery diarrhea, fever, loss of appetite, nausea and belly pain or tenderness. Because ordinary diarrhea can occasionally have some of these features too, persistent or worsening symptoms deserve a call to your clinician rather than guesswork.

Signs that need urgent medical care

Most C. diff infections are treatable, but severe cases can lead to dehydration, a dangerously swollen colon (toxic megacolon), a hole in the bowel wall (perforation) or a bloodstream infection (sepsis). These complications can progress quickly, and they are the reason certain symptoms need prompt attention.

Dehydration

Watery diarrhea can drain fluid and salts. Warning signs include very dark urine or passing little urine, a dry mouth, dizziness when standing, a racing heart, extreme weakness and confusion. Older adults, frail people and young children can become dehydrated more easily, so they should contact a clinician the same day rather than waiting. Being unable to keep any fluids down, with very little or no urine and extreme weakness, needs emergency care.

Severe belly problems

A swollen, hard or rigid belly, severe constant pain, or sudden worsening of pain after diarrhea has been present can suggest a serious complication such as toxic megacolon or perforation. A sudden stop in stool and gas together with a bloated, painful belly also needs emergency evaluation. Fever with diarrhea and belly pain or tenderness needs same-day contact with a clinic or urgent care, and emergency care if the pain becomes severe, the belly becomes hard or swollen, or you feel faint or confused.

Signs of a body-wide infection

High fever with shaking chills, a fast heartbeat, fast breathing, fainting, or new confusion can be signs of sepsis. These need emergency care.

Blood in the stool

Any blood in the stool, or black, tarry stool, needs same-day medical evaluation even if it seems small. Heavy bleeding, or any bleeding with lightheadedness or fainting, needs emergency care.

How C. diff is diagnosed and treated

If your clinician suspects C. diff, they usually order a stool test. The testing approach varies between laboratories, and results are interpreted alongside your symptoms, since some people carry the bacterium without being ill. The Infectious Diseases Society of America and the Society for Healthcare Epidemiology of America address testing and treatment for clinicians in their Clinical Practice Guideline Update on Management of Clostridioides difficile Infection in Adults (2021 focused update).

Treatment generally involves an antibiotic that targets C. diff, and your clinician will also consider whether the antibiotic that started the problem can be changed or stopped. Please do not stop or change a prescribed antibiotic on your own. Some infections need the full course, and your prescriber is the right person to weigh that decision. Medicines that slow the bowel, such as loperamide (Imodium), are generally avoided when C. diff is possible, and over-the-counter loperamide labeling advises against use with fever or bloody stool. Bismuth subsalicylate (Pepto-Bismol) contains a salicylate and is not suitable for some people, including children and teens and anyone allergic to aspirin or other salicylates. Check with your clinician or pharmacist before using either, and do not give them to young children without medical advice.

C. diff can return after treatment. The CDC notes that recurrence is a recognized problem, so tell your clinician if watery diarrhea comes back after you improved. Options for repeated infection exist, and your clinician can discuss which are appropriate.

Caring for mild diarrhea at home

• Drink fluids regularly. Water, broth and oral rehydration solutions can help replace what you lose.
• Eat what you can tolerate. Bland foods such as rice, bananas, toast and soup are often easier on the stomach. Very greasy or sugary foods may worsen loose stools in some people.
• Keep taking your antibiotic as prescribed unless your prescriber tells you otherwise. Taking some antibiotics with food may reduce stomach upset, but check your label or ask your pharmacist.
• Tell your clinician if diarrhea is frequent or lasts more than a few days, or if you are unsure, so they can decide whether testing is appropriate.
• Wash hands with soap and water. The CDC notes that alcohol-based sanitizer is not reliable against C. diff spores. If C. diff is possible, cleaning bathroom surfaces with a bleach-containing product is generally recommended.

Some people ask about probiotics. Evidence is mixed and depends on the product and the person, so see our article on probiotics and antibiotics and talk with your clinician, particularly if you are older, pregnant or have a weakened immune system.

Reducing your chances next time

The most effective step is using antibiotics only when they are likely to help. Viral illnesses such as colds and flu do not respond to them. If you are prescribed one, ask your clinician whether it is needed and what to watch for. Tell any new clinician or dentist that you have had C. diff, since that history may influence future antibiotic choices.

Take the antibiotic exactly as directed and do not share or save leftovers. If you develop watery diarrhea in the weeks after any antibiotic, mention the recent course when you call, even if it ended a while ago.

When to Seek Medical Care