Which Medicines Make Stool Loose? A Guide to Common Culprits

October 12, 2025 •

5 min read

Approximately 7% of all adverse drug reactions involve the gastrointestinal tract, with diarrhea being a common symptom. Various medications, from over-the-counter treatments to prescription drugs for chronic conditions, can make stool loose, and understanding the root cause is the first step toward effective management.

Quick Summary

Several types of medications can lead to loose stools, including various laxatives, antibiotics that disrupt gut flora, diabetes treatments like metformin, and magnesium-containing antacids. Managing this common side effect involves adjusting diet, staying hydrated, and consulting a healthcare provider before altering a medication regimen.

Key Points

Diverse Causes of Loose Stools: It's not just laxatives; many common medications like antibiotics, metformin, and antacids can also cause loose stools due to various mechanisms.
Antibiotics Disrupt Gut Flora: This is one of the most common causes of drug-induced diarrhea, resulting from the killing of beneficial gut bacteria along with the target pathogens.
Not all Laxatives are the Same: Laxatives like osmotics and stimulants have different effects on the bowel and are more likely to cause loose stools than gentler stool softeners or bulk-forming agents.
Medication-Induced Diarrhea Varies: The severity can range from a mild, self-limiting issue to a more serious infection, such as C. difficile, which requires specific treatment.
Manage with Hydration and Diet: Simple dietary changes, including focusing on electrolyte-rich fluids and bland foods, are key to managing symptoms, but serious cases require professional medical advice.
Consult a Doctor Before Stopping Meds: Never stop or change the dosage of a prescribed medication without consulting your doctor, as they can help determine the best course of action and rule out more serious issues.
Look for Warning Signs: Be aware of red flags like bloody stools, fever, or signs of dehydration, and seek immediate medical attention if they appear.

Medications That Cause Loose Stools: A Comprehensive Guide

Loose stools, or diarrhea, can be an uncomfortable and sometimes concerning side effect of many medications. While laxatives are designed specifically to induce bowel movements, many other drugs can have the same effect unintentionally through different mechanisms. This comprehensive guide details the common classes of drugs that cause loose stools, explains their physiological actions, and provides practical advice for managing the symptoms.

The Most Obvious Culprits: Laxatives and Stool Softeners

This class of medication is designed specifically to relieve constipation, and their primary function is to make stool softer or promote bowel movements. Overusing or misusing them, however, can easily lead to overly loose or watery stools. They work in several distinct ways:

Osmotic Laxatives: These draw water from the surrounding tissues into the colon. This influx of water softens the stool and increases its bulk, making it easier to pass. Common examples include polyethylene glycol (MiraLAX) and magnesium hydroxide (Milk of Magnesia). High doses can lead to cramping and watery diarrhea.
Stimulant Laxatives: These act on the nerves in the intestinal lining to cause rhythmic muscle contractions, effectively forcing stool out. Because they stimulate bowel activity, they can be harsher on the digestive system and are more likely to cause cramping and diarrhea, especially at higher doses. Common ingredients include bisacodyl and senna.
Stool Softeners (Emollients): Unlike stimulants, these are milder and work by adding moisture and fat to the stool to soften it, reducing the need for straining. Docusate sodium is a common active ingredient. It may take a couple of days to be effective, and while less likely, can still cause loose stools.
Bulk-Forming Laxatives: These are typically fiber supplements that absorb water and form a soft, bulky gel in the intestine. The increased bulk stimulates the natural muscle contractions of the colon. They are generally considered the gentlest, but not drinking enough water with them can actually worsen constipation.

Antibiotics: Unbalancing the Gut's Ecosystem

Antibiotics are one of the most common causes of drug-induced diarrhea, accounting for approximately 25% of cases. Their mechanism of action is both simple and complex. By killing the pathogenic bacteria causing an infection, they also disrupt the delicate balance of beneficial bacteria in the gut. This imbalance can lead to several scenarios:

Mild to Moderate Diarrhea: The shift in gut flora simply causes temporary digestive upset, which often resolves after the course of antibiotics is finished.
C. Difficile Infection: A more serious issue occurs when the balance is thrown off enough to allow the bacteria Clostridioides difficile (C. diff) to overgrow and produce toxins. This can lead to severe, watery, and sometimes bloody diarrhea.

Many types of antibiotics can cause this issue, with some of the most commonly cited being penicillin, cephalosporins, and clindamycin.

Medications for Chronic Conditions with an Unwanted Side Effect

It can be surprising to discover that drugs prescribed for other health issues can have a laxative effect. Their mechanisms vary widely, from altering gut motility to interfering with nutrient absorption.

Metformin: A cornerstone medication for Type 2 diabetes, metformin is notorious for causing gastrointestinal side effects, including diarrhea, especially when treatment begins. It's thought to affect the way the body absorbs sugar and interacts with digestive functions.
Antacids and Proton Pump Inhibitors (PPIs): Some antacids, particularly those containing magnesium compounds like magnesium hydroxide (Milk of Magnesia), have a direct osmotic effect. Proton pump inhibitors (PPIs) used for heartburn and acid reflux, such as omeprazole and esomeprazole, have been associated with an increased risk of C. difficile infection and diarrhea, especially with long-term use.
Antidepressants: Selective Serotonin Reuptake Inhibitors (SSRIs) can increase gastrointestinal motility by affecting serotonin levels throughout the body. This can lead to a faster passage of food and, consequently, loose stools.
NSAIDs: Long-term use of nonsteroidal anti-inflammatory drugs like ibuprofen and naproxen can irritate the stomach and intestinal lining, leading to digestive issues and diarrhea.
Chemotherapy Drugs: Chemotherapy is an aggressive treatment that often leads to diarrhea by causing damage and inflammation to the intestinal mucosa.
ACE Inhibitors: These blood pressure medications, like ramipril, have been linked to diarrhea as a potential side effect.
Magnesium and Calcium Supplements: Similar to antacids, high doses of magnesium can cause an osmotic laxative effect.

Comparative Overview of Medications Causing Loose Stools


Drug Class	Example Medications	Primary Mechanism	Management Considerations
Osmotic Laxatives	Polyethylene glycol, Milk of Magnesia	Draw water into the colon, softening stool.	Monitor dosage; can cause cramping and dehydration.
Stimulant Laxatives	Bisacodyl, Senna	Force intestinal muscle contractions.	Use short-term only; risk of dependency and cramping.
Stool Softeners	Docusate sodium	Increase water and fat absorption in stool.	Gentle, takes time to work; intended for short-term use.
Antibiotics	Penicillins, Cephalosporins	Disrupt gut bacteria, potentially allowing overgrowth of harmful strains.	Probiotics may help; monitor for C. diff symptoms.
Metformin	(Type 2 diabetes)	Alters nutrient absorption and digestive function.	Side effects often decrease over time; dose adjustments may be needed.
Antacids (Magnesium)	Magnesium Hydroxide	Osmotic effect, drawing fluid into the bowel.	Switch to a magnesium-free antacid if needed.
Antidepressants (SSRIs)	Sertraline, Fluoxetine	Increase gastrointestinal motility.	Discuss dosage or alternative medications with a doctor.
NSAIDs	Ibuprofen, Naproxen	Can irritate intestinal lining, especially with long-term use.	Take with food or consider a different pain reliever.
Chemotherapy Drugs	(Various)	Damage intestinal mucosa and alter gut flora.	Supportive care is often needed; consult with an oncologist.

Managing Medication-Induced Diarrhea

If you suspect a medication is causing loose stools, here are some strategies, but always consult a healthcare provider before making any changes.

Stay Hydrated: Drink plenty of fluids, especially those with electrolytes, like broths, sports drinks, or oral rehydration solutions. Dehydration is a significant risk with persistent loose stools.
Adjust Your Diet: Follow the BRAT diet (Bananas, Rice, Applesauce, Toast) which consists of low-fiber, bland foods that can help firm up stools. Avoid foods that can worsen diarrhea, including spicy, greasy, or high-fiber items, caffeine, and alcohol.
Timing Your Medication: For some drugs like Metformin, taking it with food can help reduce stomach upset. A doctor may also suggest taking it at a different time of day.
Probiotics: In cases of antibiotic-induced diarrhea, probiotics may help restore the balance of good bacteria in your gut.
Over-the-Counter Antidiarrheals: A product like loperamide (Imodium) might be recommended, but it's crucial to consult a doctor first, especially if an infection like C. difficile is a possibility.

When to Seek Medical Advice

While mild loose stools may resolve on their own, more serious symptoms require medical attention. Contact a healthcare provider if you experience any of the following:

Bloody or black, tarry stools
Severe abdominal cramping
Fever
Signs of dehydration (decreased urination, dizziness)
The diarrhea persists for more than a few days, even after trying dietary adjustments

Conclusion

Many factors can influence bowel movements, and medication is a major one. From the intentional effects of laxatives to the unintended consequences of antibiotics and treatments for chronic diseases, a wide range of drugs can cause loose stools. Patients who experience this side effect should focus on proper hydration and dietary adjustments while consulting a healthcare provider before stopping or changing any medication. A doctor can help determine the best course of action, which may involve a dosage adjustment, switching to an alternative drug, or prescribing symptomatic relief. Understanding the link between your medications and your digestive health is essential for overall well-being. For more information, you can read about antibiotic-associated diarrhea on the Mayo Clinic website.

Frequently Asked Questions (FAQs)

Frequently Asked Questions

Yes, antibiotics are a very common cause of loose stools. They can disrupt the natural balance of bacteria in your gut, leading to diarrhea. In more severe cases, it can lead to an overgrowth of a harmful bacterium called C. difficile.

Metformin, a medication for Type 2 diabetes, commonly causes loose stools and other gastrointestinal side effects. While the exact reason is not fully understood, it is believed to be related to the way the drug affects nutrient absorption and digestive processes.

Yes, certain heartburn medications can cause loose stools. Antacids that contain magnesium can have an osmotic effect, drawing water into the bowels. Additionally, long-term use of proton pump inhibitors (PPIs) has been linked to an increased risk of C. difficile infection, which causes diarrhea.

Yes, while stool softeners are a type of laxative, they are generally much milder. Traditional stimulant laxatives work by forcing muscle contractions and are more likely to cause overly loose or watery stools. Stool softeners like docusate simply add moisture to the stool to make it easier to pass, but can still lead to loose stools, especially if overused.

If your medication is causing loose stools, you should first focus on staying hydrated, particularly with electrolyte-rich fluids. You can also try a bland diet (like the BRAT diet). It is crucial to consult your healthcare provider before stopping or changing the dose of your medication.

Besides antibiotics and metformin, other medications include certain antidepressants (SSRIs), NSAIDs like ibuprofen, chemotherapy drugs, and certain heart medications like ACE inhibitors. High doses of magnesium and calcium supplements can also have this effect.

You should contact your doctor if the loose stools are severe or persistent, if you see blood in your stool, if you develop a fever, or if you show signs of dehydration like decreased urination or light-headedness. These could indicate a more serious issue that needs attention.