What Medications Cause Diarrhea? Types and Warning Signs

Diarrhea is one of the most common side effects of medication use, and it can be triggered by drugs in nearly every class, from everyday antibiotics and antacids to specialized cancer therapies and blood pressure pills. The mechanisms range from straightforward osmotic effects to subtle immune reactions that damage the intestinal lining over months or years. Understanding which medications are most likely to cause this problem, and knowing the warning signs that distinguish mild nuisance from something dangerous, can spare you unnecessary testing and help you have a more productive conversation with your doctor.

How Medications Cause Diarrhea

Drug-induced diarrhea is not a single problem with a single explanation. Several distinct mechanisms are at work depending on the drug involved: osmotic diarrhea, where a substance pulls extra water into the intestine; secretory diarrhea, where the intestinal lining actively pumps out fluid; shortened transit time, where the gut moves contents through too fast for water to be reabsorbed; mucosal damage, where the drug injures the intestinal lining directly; and malabsorption, where fats or carbohydrates are not properly digested.¹ Some drugs even cause diarrhea indirectly by slowing the gut down so much that bacteria overgrow in the small intestine, producing bile salt changes that trigger loose stools.²

A single medication can involve more than one of these pathways simultaneously, and two people taking the same drug may experience it differently based on their genetics, diet, and what other medications they are on. That complexity is part of why drug-induced diarrhea is often misdiagnosed as irritable bowel syndrome or a dietary problem.

Antibiotics

Antibiotics are the single most recognized cause of drug-induced diarrhea. The reason is straightforward: antibiotics kill off beneficial gut bacteria along with the targeted infection, disrupting the microbial balance that normally keeps digestion running smoothly. Most cases of antibiotic-associated diarrhea are mild and self-limiting, resolving once the course of treatment ends.³

The serious concern, though, is Clostridioides difficile (C. diff) infection. When antibiotics wipe out competing bacteria, C. diff can colonize the colon and produce toxins that cause inflammation. The resulting disease ranges from frequent watery diarrhea to full-blown colitis, toxic megacolon, and in severe cases, death.³ Broad-spectrum antibiotics like fluoroquinolones and clindamycin carry the highest risk, though virtually any antibiotic can trigger it. Fever, bloody stool, and severe abdominal pain during or shortly after an antibiotic course should prompt immediate medical evaluation.

Metformin and Diabetes Medications

Metformin, the most widely prescribed drug for type 2 diabetes, is notorious for causing gastrointestinal symptoms. Loose stools, cramping, and nausea affect a substantial fraction of people who start taking it. The mechanism is not simply that metformin “upsets the stomach.” It alters the gut microbiome, increases intestinal glucose levels, and disrupts bile salt reabsorption in the small intestine, all of which contribute to diarrhea.⁴ Specifically, metformin increases the bile acid pool in the intestine by reducing absorption in the ileum, which can change stool consistency and feed shifts in microbial populations.⁵

What makes metformin-related diarrhea particularly tricky is that it can appear years after a person has been on a stable dose with no prior problems. One documented case involved a patient who developed nausea, cramping, and explosive watery diarrhea after several years of stable metformin therapy. The diarrhea was occasionally severe enough to cause incontinence, and all symptoms resolved completely when the medication was stopped.⁶ Because the onset can be so far removed from when the drug was first prescribed, patients and clinicians often fail to make the connection. Some patients endure years of testing for other conditions before anyone thinks to trial a metformin break.⁴

Proton Pump Inhibitors

Proton pump inhibitors like omeprazole and lansoprazole are among the most commonly used medications worldwide, prescribed for acid reflux and ulcers and also widely available over the counter. Most people tolerate them without issues, but a growing body of evidence links PPIs to microscopic colitis, a condition where the colon looks normal on standard imaging but biopsy reveals chronic inflammation that causes persistent watery diarrhea.⁷

Among the PPIs, lansoprazole has attracted the most attention. It is thought to inhibit proton pumps in the colon itself, promoting inflammation and diarrhea in that specific location.⁸ The challenge is that PPIs are so commonly used, and microscopic colitis develops so gradually, that the connection is easy to miss. If you have been on a PPI for months and develop unexplained chronic watery diarrhea, the medication deserves consideration as a possible cause.

Blood Pressure Medications

Most blood pressure drugs are well tolerated, but one particular medication, olmesartan, has been linked to a distinctive and serious form of intestinal damage that closely mimics celiac disease. Patients develop chronic diarrhea, weight loss, and nausea, and biopsies of the small intestine reveal villous atrophy and inflammatory cells that look virtually identical to what you would see in someone with untreated celiac disease.⁹

This condition, called olmesartan-associated enteropathy, can be severe. One reported case involved a 69-year-old woman who experienced 20 watery bowel movements per day for a year and lost 10 kilograms. Capsule endoscopy revealed villous atrophy throughout her small bowel. After olmesartan was stopped, her diarrhea improved and repeat imaging confirmed that the intestinal damage was healing.¹⁰ The key diagnostic clue is that celiac blood tests come back negative despite biopsy findings that scream celiac disease.¹¹ Other blood pressure drugs in the same class (angiotensin receptor blockers) have not shown this pattern to any meaningful degree. If you are on olmesartan and have unexplained chronic diarrhea, bringing this specific concern to your gastroenterologist could save months of diagnostic workup.

Cancer Treatments

Diarrhea is one of the most common and sometimes most dangerous side effects of cancer therapy, and it shows up with both traditional chemotherapy and newer immunotherapies, though through very different mechanisms.

Chemotherapy Drugs

Among conventional chemotherapy agents, irinotecan is particularly well known for causing severe diarrhea. Irinotecan is metabolized in the body to an active form called SN-38, and the enzymes involved in that conversion vary from person to person based on genetic differences. People with certain genetic variants of the enzyme UGT1A1 process the drug differently, leading to higher levels of the active metabolite in the gut and a greater likelihood of diarrhea.¹² Chemotherapy-induced diarrhea in general can be severe enough to require hospitalization, and oncology guidelines treat it as a potentially life-threatening complication that needs proactive management rather than an afterthought.

Immune Checkpoint Inhibitors

Newer cancer immunotherapies work by releasing the brakes on the immune system so it can attack tumors more aggressively. The trade-off is that an unleashed immune system sometimes attacks healthy tissue too, and the gut is one of the most common targets. Diarrhea and colitis are among the most frequent immune-related adverse events from these drugs.¹³

The rates vary substantially by drug type. A meta-analysis found that PD-1/PD-L1 inhibitors caused diarrhea in about 10% of patients, while the CTLA-4 inhibitor ipilimumab caused it in roughly a third. Combining ipilimumab with another checkpoint inhibitor pushed rates even higher.¹⁴ The colitis that develops can range from mild to life-threatening, and the incidence of immune-mediated colitis varies from roughly 1% to 25% depending on the specific drug and whether combination therapy is used.¹⁵ Patients on these medications are generally counseled to report any new diarrhea promptly, because early treatment with immunosuppressive agents can prevent the colitis from escalating.

Antidepressants

Selective serotonin reuptake inhibitors (SSRIs) are widely prescribed for depression and anxiety, and diarrhea is a well-recognized side effect, especially with sertraline. Serotonin is not just a brain chemical; the gut contains enormous amounts of it, and serotonin plays a central role in regulating intestinal motility and fluid secretion. When an SSRI blocks serotonin reuptake in the gut, serotonin levels rise in the intestinal wall, which can speed things up and trigger loose stools.

Sertraline stands out among the SSRIs for causing the most diarrhea and nausea. Research suggests this is because sertraline is substantially more potent than other antidepressants at inhibiting serotonin reuptake and also affects dopamine transporters in the gut. Dopamine in the colon can promote inflammatory responses that contribute to diarrhea.¹⁶ For many people, SSRI-related diarrhea improves over the first few weeks as the body adjusts. When it persists, switching to a different antidepressant or adjusting the dose often helps.

Immunosuppressants After Transplantation

People who receive organ transplants take immunosuppressive drugs to prevent rejection, and several of these medications affect the gut. Mycophenolate mofetil, one of the most commonly used transplant drugs, is a frequent cause of chronic diarrhea. It is considered the most common reason for villous atrophy in the small intestine among solid organ transplant patients who develop persistent loose stools.¹⁷ Because transplant patients are already medically complex, and because diarrhea in this population can also signal infection or graft-versus-host disease, working out the cause usually requires close collaboration between the transplant team and gastroenterology.

Over-the-Counter Products and Hidden Ingredients

Not all drug-induced diarrhea comes from prescription medications. Several common over-the-counter products and inactive ingredients can cause problems that people do not think to attribute to a pill.

Magnesium-containing antacids are a classic culprit. Magnesium salts draw water into the intestine through osmosis, and at high enough doses they function essentially as a laxative.² People who take magnesium supplements for muscle cramps or heart health sometimes push the dose high enough to cross into this territory without realizing it.

Sugar alcohols like sorbitol, xylitol, and mannitol are another frequently overlooked cause. These are used as sweeteners in sugar-free candies, gums, and lozenges, but they also appear as inactive ingredients in many liquid medications. They cause osmotic diarrhea by pulling water into the intestine, and while the diarrhea is not harmful to the intestinal lining, it can be significant enough to interfere with daily life.¹⁸ Sorbitol in particular is used as an excipient in some liquid pharmaceutical preparations, meaning the diarrhea may be caused not by the active drug but by the sweetener it is dissolved in.² If you develop diarrhea after switching from a tablet to a liquid form of the same medication, the inactive ingredients are worth investigating.

Warning Signs That Need Medical Attention

Most drug-induced diarrhea is an inconvenience, not an emergency. But certain features signal that something more serious is happening and warrant prompt evaluation:

Blood or mucus in the stool: This can indicate colitis or mucosal damage rather than simple osmotic diarrhea, and it changes the urgency of the workup.
Fever: Diarrhea plus fever raises the possibility of C. diff infection or another infectious cause, especially during or after antibiotic use.
Significant weight loss: Unintentional weight loss alongside chronic diarrhea suggests malabsorption, which can occur with drugs like olmesartan or mycophenolate that damage the small intestinal lining.
Signs of dehydration: Dizziness, dark urine, rapid heart rate, and dry mouth indicate that fluid losses are outpacing intake, which is especially dangerous in older adults and young children.
Very high stool frequency: More than six to eight watery stools per day, or any diarrhea severe enough to limit your normal activities, crosses the threshold from mild to moderate or severe.

A thorough medication history is one of the most important tools in evaluating chronic diarrhea. The link between a drug and the diarrhea can be difficult to establish when the onset is delayed by months or years, and patients sometimes undergo extensive testing for other conditions before the medication is identified as the cause.¹

Why Older Adults Are More Vulnerable

Older adults face a disproportionately high risk of drug-induced diarrhea for several converging reasons. They tend to take more medications simultaneously, which increases both the raw probability that one of those drugs will cause diarrhea and the likelihood of interactions between drugs that compound the effect. Age-related changes in kidney and liver function alter how drugs are metabolized and cleared, potentially raising drug levels to ranges where gastrointestinal side effects become more likely.²

The consequences are also more serious. Dehydration from diarrhea progresses faster in older adults, who may already have reduced fluid reserves and impaired thirst sensation. Electrolyte imbalances from prolonged diarrhea can cause confusion, muscle weakness, and cardiac arrhythmias. When an older person develops new diarrhea, a systematic review of every medication on the list, including over-the-counter products and supplements, is an important early step before pursuing invasive diagnostic testing.

When the Timing Does Not Fit the Obvious Pattern

One of the most common reasons drug-induced diarrhea goes unrecognized is that clinicians and patients both expect the side effect to appear shortly after starting or increasing a medication. Many drug reactions do follow that pattern. But some of the most clinically significant cases break it completely.

Metformin can cause late-onset chronic diarrhea years into stable therapy, as described earlier. Olmesartan-induced enteropathy often develops after prolonged use, sometimes manifesting only after a patient has been on the drug for a year or more.¹⁰ PPIs and microscopic colitis follow a similarly delayed timeline. In all these cases, the long gap between starting the medication and developing symptoms makes the connection counterintuitive. A person is far more likely to suspect a new food, a recent illness, or stress than a medication they have tolerated without complaint for years.

For clinicians, the practical lesson from these delayed-onset patterns is that a drug history review should not stop at “what medications were recently started or changed.” It should also ask what medications the patient has been on for years, especially those with known associations with delayed enteropathy or colitis. For patients, the takeaway is that a medication does not get a permanent pass just because it did not cause problems in the first few months. If chronic diarrhea develops without a clear explanation, raising the question of a drug cause with your doctor is always reasonable, even for medications you have been taking for a long time.⁶

1
PubMed. Drug-induced diarrhoea
2
PubMed. Mechanisms of drug-induced diarrhoea in the elderly
3
PubMed Central. Clostridium difficile infection and antibiotic-associated diarrhoea
4
PubMed Central. Metformin-Induced Chronic Diarrhea Misdiagnosed as Irritable Bowel Syndrome for Years
5
PubMed Central. Metformin and the gastrointestinal tract
6
PubMed. Metformin as a cause of late-onset chronic diarrhea
7
PubMed. Association Between Proton Pump Inhibitors and Microscopic Colitis
8
PubMed. Proton-Pump Inhibitors Are Associated With an Increased Risk of Microscopic Colitis: A Population-Based Study and Review of the Literature
9
PubMed Central. Olmesartan-Induced Enteropathy
10
PubMed. Diagnosis and resolution of olmesartan-associated sprue-like enteropathy confirmed by capsule endoscopy: a case report and literature review
11
PubMed Central. Severe Sprue-Like Enteropathy and Colitis due to Olmesartan: Lessons Learned From a Rare Entity
12
European Journal of Pharmacology. Mechanisms and emerging strategies for irinotecan-induced diarrhea
13
PubMed Central. Immune checkpoint inhibitor-induced diarrhea and colitis: an overview
14
PubMed. Immune checkpoint Inhibitor-Induced diarrhea and Colitis: Incidence and Management. A systematic review and Meta-analysis
15
PubMed Central. Immune checkpoint inhibitor-induced colitis: A comprehensive review
16
Therapeutics and Clinical Risk Management. Risks of Digestive System Side-Effects of Selective Serotonin Reuptake Inhibitors in Patients with Depression: A Network Meta-Analysis
17
PubMed Central. Small Bowel Severe Enteropathy From Mycophenolate Mofetil
18
PubMed Central. Gastrointestinal Disturbances Associated with the Consumption of Sugar Alcohols with Special Consideration of Xylitol: Scientific Review and Instructions for Dentists and Other Health-Care Professionals