Researchers tested whether probiotics—beneficial bacteria supplements—could prevent diarrhea in cancer patients undergoing chemotherapy. They studied 28 patients with gastrointestinal cancer, giving half a probiotic supplement and half a placebo for 90 days. Surprisingly, the probiotic group didn’t experience significantly fewer or less severe diarrhea episodes than the placebo group. While probiotics are generally safe, this study suggests they may not be effective for preventing this particular chemotherapy side effect, though more research with larger groups of patients is needed.

The Quick Take

  • What they studied: Whether taking probiotic supplements could prevent or reduce diarrhea in cancer patients receiving chemotherapy drugs
  • Who participated: 28 patients with gastrointestinal cancer who were about to start chemotherapy treatment. Half received probiotics and half received a placebo (fake pill) for 3 months
  • Key finding: Probiotics did not significantly reduce diarrhea compared to placebo. About 64% of the probiotic group experienced diarrhea versus 71% in the placebo group—a difference too small to be meaningful
  • What it means for you: If you’re undergoing chemotherapy, probiotics may not be a reliable way to prevent diarrhea. Talk to your doctor about other proven strategies for managing this side effect. The good news: probiotics appeared safe with no infections reported

The Research Details

This was a randomized controlled trial, which is considered one of the strongest types of medical research. Researchers randomly assigned 28 cancer patients to two equal groups: one received a probiotic supplement containing five different beneficial bacteria strains (20 billion live bacteria cells per dose), while the other group received a placebo that looked identical but contained no active ingredients. Both groups took their assigned product once daily for 90 days while undergoing chemotherapy. Patients kept detailed diaries of their bowel movements using a standardized scale to track diarrhea severity and frequency.

The study was conducted between April 2022 and June 2024 at a medical center treating gastrointestinal cancer patients. All participants received the same types of chemotherapy drugs (fluoropyrimidine, oxaliplatin, and/or irinotecan), which are known to commonly cause diarrhea. This consistency helped ensure that any differences between groups would be due to the probiotic, not different treatments.

This research design is important because it reduces bias—neither patients nor researchers knew who received the real probiotic versus placebo until the study ended. This ‘blinding’ prevents expectations from influencing results. The randomization ensures the two groups were similar at the start, making fair comparisons possible. Tracking bowel habits with a standardized scale provides objective data rather than relying on patient memory.

Strengths: This was a properly designed randomized controlled trial with blinding, which is a high-quality research method. The probiotic strains used are well-established and safe. Limitations: The study was quite small (only 28 patients), which limits confidence in the results. Larger studies might find different results. The study was relatively short (90 days), so long-term effects are unknown. Results from one medical center may not apply everywhere.

What the Results Show

The main finding was that probiotics did not prevent or reduce chemotherapy-related diarrhea. In the probiotic group, 44% experienced moderate to severe diarrhea compared to 56% in the placebo group—a small difference that could easily be due to chance. When looking at any diarrhea (mild or severe), 64% of the probiotic group had it versus 71% in the placebo group. The researchers used statistical testing to determine if these differences were meaningful, and they were not.

The median number of diarrhea episodes over the 90-day period was slightly lower in the probiotic group (8 episodes) compared to placebo (9 episodes), but this small difference was not statistically significant. When researchers looked at specific subgroups—such as patients receiving different chemotherapy drugs or those with different baseline health conditions—they still found no benefit from probiotics in any group.

An important positive finding was safety: no infections or adverse effects related to the probiotic strains were detected in either group. This suggests that if someone chooses to take these probiotics, they appear unlikely to cause harm. The study also confirmed that diarrhea is indeed very common during this type of chemotherapy, occurring in about two-thirds of patients regardless of probiotic use.

Previous research suggested that gut bacteria might play a role in chemotherapy-related diarrhea, which made probiotics seem like a logical treatment. However, this study adds to growing evidence that simply adding beneficial bacteria through supplements may not be enough to prevent this side effect. The gut microbiome is complex, and chemotherapy damages it in multiple ways that probiotics alone may not address.

The biggest limitation is the small number of participants (28 patients). With such a small group, it’s harder to detect real effects if they exist. A study with hundreds of patients might show different results. The study only lasted 90 days, so we don’t know about longer-term effects. The study was conducted at one medical center, so results might differ in other populations or healthcare settings. The study didn’t measure changes in gut bacteria directly, so we don’t know if the probiotics actually colonized the gut as intended.

The Bottom Line

Based on this research, probiotics cannot be recommended as a proven way to prevent chemotherapy-related diarrhea. However, they appear safe if someone wants to try them. More effective strategies may include: staying well-hydrated, eating bland foods, avoiding high-fiber foods during treatment, and discussing anti-diarrheal medications with your oncologist. This is low-confidence evidence against probiotics for this specific use.

This research is most relevant to people with gastrointestinal cancer undergoing chemotherapy. It’s less clear whether results apply to other cancer types or other causes of diarrhea. People considering probiotics for other health reasons should discuss this with their doctor, as this study only addresses one specific use.

If diarrhea develops during chemotherapy, it typically appears within the first few weeks of treatment. Preventing it from the start (as this study attempted) would be ideal, but this research suggests probiotics may not help with that prevention. If diarrhea develops despite probiotics, other treatments should be discussed with your medical team.

Want to Apply This Research?

  • Track daily bowel movement frequency and consistency using the Bristol Stool Scale (a simple 1-7 scale where 1 is very hard and 7 is watery). Record this daily during chemotherapy to identify patterns and share objective data with your healthcare team
  • If using the app during cancer treatment, log your probiotic intake (if taking them) alongside bowel movement data to help your doctor assess whether they’re helping you personally. This individual tracking is more useful than population studies since everyone responds differently
  • Create a 90-day tracking period aligned with your chemotherapy schedule. Monitor hydration levels, dietary changes, and any medications used for diarrhea management alongside probiotic use. Share this data with your oncology team monthly to adjust your management strategy as needed

This research suggests probiotics may not effectively prevent chemotherapy-related diarrhea, but it does not mean probiotics are harmful. If you are undergoing cancer treatment or considering probiotics, consult with your oncologist or gastroenterologist before starting any supplement. This study involved a small number of patients, and individual responses may vary. Do not stop or change your cancer treatment based on this information. Diarrhea during chemotherapy can be serious—always report it to your medical team so they can recommend proven management strategies.