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Probiotics, Yogurt and Gut Bacteria: What Is Known About Cancer Risk and Treatment

Gut bacteria are linked to colorectal cancer and to how some people respond to immunotherapy. Does that mean a probiotic or yogurt helps? Here is what studies show, what they do not, and what to ask your care team.

By Cancer Explained Editorial TeamPublished

Original commentary from the Cancer Explained editorial team.

Review status: AI-assisted, source checked. Not reviewed by a clinician. How we check pages

Sources last checked .

Small ring-shaped bacterial colonies on a dark blue culture surface.
Small bacterial colonies on a culture plate. Centers for Disease Control and Prevention, via Wikimedia Commons (Public domain)

Please note: this page is educational only — it is not medical advice, and it does not speculate about anyone’s health beyond reliable public reporting. For questions about your own health, talk with your healthcare team.

The short answer

Gut bacteria and cancer are linked in research. But the link does not yet tell us that taking a probiotic or eating yogurt lowers cancer risk or helps treatment.

The studies we could open show three things. Some gut bacteria are found more often in colorectal tumors. Gut bacteria seem to affect how some people respond to immunotherapy. And probiotics for treatment side effects have mixed results. One study in people on immunotherapy raised a worry that probiotics could do harm.

If you are in cancer treatment, ask your care team before you take any probiotic.

The idea as it circulates

The idea goes like this. Your gut holds trillions of bacteria. Cancer is linked to changes in those bacteria. So "good" bacteria from yogurt or a probiotic capsule must protect you, or help your treatment work better.

Part of that is true. The gut community of bacteria, called the microbiome, is a real area of cancer research. The jump from "linked" to "a supplement helps" is where the evidence gets thin.

What the studies show: gut bacteria and colorectal cancer

In 2012, researchers looked at 95 pairs of colorectal tumor and normal tissue. They found more Fusobacterium bacteria in the tumors. The authors wrote that the exact role of these bacteria in the cancer "requires further investigation" (Castellarin 2012).

A 2023 review says studies in humans and in animal models point to a role for some gut bacteria in how colorectal cancer starts and grows. It names Fusobacterium nucleatum, a toxin-making strain of Bacteroides fragilis, and E. coli that makes a substance called colibactin (review, 2023).

How strong is this? Finding a germ in a tumor shows an association. It does not show that the germ caused the cancer. The germ could move in after the tumor forms. Many of the studies are in mice. That kind of work is useful for generating ideas, but mice are not people.

What the studies show: gut bacteria and immunotherapy

Immunotherapy called checkpoint inhibitors helps the immune system attack cancer. A 2018 study found that patients who took antibiotics had less benefit from these drugs. In mice, stool transplants from patients who responded helped the treatment work better. Stool from patients who did not respond did not. The study also found that a bacterium called Akkermansia muciniphila was more common in people who responded (Routy 2018).

This is a link, plus mouse experiments. It does not say that a bacterium in a capsule would give the same result in a person.

What the studies show: probiotics during treatment

Diarrhea from radiation or chemotherapy. The NCCIH, part of the National Institutes of Health, says that for diarrhea caused by cancer treatment, a 2018 review of 12 studies (1,554 people) found the evidence for a benefit of probiotics "inconclusive." That is the Cochrane review. Its authors said the evidence was low or very low certainty, that all studies were small and not alike, and that no study reported serious side effects (Cochrane 2018).

The trials point in different directions.

  • In a Canadian trial of 246 people getting pelvic radiation, overall diarrhea (grade 2 to 4) did not differ in a statistically significant way (p = 0.13). At 60 days, 35% of the standard-dose group had no moderate or severe diarrhea, compared with 17% of the placebo group (Demers 2014).
  • In a trial of 85 women getting pelvic radiation, a probiotic yogurt drink did not lower grade 2 or higher diarrhea. Grade 2 or higher diarrhea or loperamide use occurred in 30 of 44 in the probiotic group and 24 of 41 in the placebo group (p = 0.568) (Giralt 2008). The trial did not find the difference to be significant, so we do not read this as proof of harm.
  • In a 2024 trial of 234 people getting pelvic radiation, grade 2 bowel injury occurred in 26% of the probiotic group and 22% of the placebo group. The authors concluded the probiotic did not prevent it (p = 0.493) (PREP trial 2024).
  • In a 150-person study of chemotherapy for colorectal cancer, people who took Lactobacillus GG had less severe diarrhea (22% vs. 37%, p = 0.027). The authors found no toxicity from the probiotic (Osterlund 2007).

So some trials show a benefit and some do not. Each trial used a different product, which matters because the NCCIH notes that one kind of probiotic may not act like another.

Possible harm with immunotherapy. A 2021 study in Science looked at 128 people with melanoma who were treated with checkpoint inhibitors. Higher fiber intake went with longer time before the cancer got worse. The authors said the benefit was most pronounced in people with enough fiber and no probiotic use. In mice, a low-fiber diet or probiotics weakened the response to anti-PD-1 treatment (Spencer 2021). The human part is an association, not a trial. But it is a reason to talk with your care team before you add a probiotic during immunotherapy.

What the studies show: infections in people with weak immune systems

The NCCIH says the risk of harmful effects from probiotics "is greater in people with severe illnesses or compromised immune systems." It lists possible harms, including infections. It also says some products have been found to contain microorganisms other than those on the label.

Cancer treatment can weaken the immune system. The reports we opened show why this matters.

  • A report described a 73-year-old on chemotherapy who got a bloodstream fungal infection in a central line while taking a Saccharomyces boulardii probiotic. The infection cleared after the probiotic was stopped (case report 2017).
  • A French review of 10 cases of this fungal bloodstream infection found that 70% of patients had a central venous line. In 6 cases, an S. boulardii probiotic had been given. Half of the patients died. The authors called it rare but life-threatening (Infect Dis Now 2021). These patients were often very ill, so the deaths cannot be blamed on the probiotic alone.
  • At a Seattle cancer center, 19 of 3,796 stem cell transplant patients (0.5%) had a bloodstream infection with organisms found in many probiotics, most often Lactobacillus. No deaths were due to these infections. The study did not check whether those patients had taken probiotics (Transpl Infect Dis 2016).

We did not find a source we could open that gives a safe or unsafe rule for people with low white blood counts. We can say the risk is real and that the people studied were often very ill.

What the studies show: yogurt and fermented foods

Yogurt is not the same as a probiotic capsule. Yogurt is a food, and not all yogurt has live cultures that are named or counted.

An Italian study followed 45,241 adults for 12 years. People who ate the most yogurt had a lower risk of colorectal cancer than those who ate the least (hazard ratio 0.65 after adjusting for other factors). The authors said larger studies were needed to rule out other explanations (Pala 2011).

A 2021 review pooled 16 studies. It found yogurt eaters had a lower colorectal cancer risk (odds ratio 0.87 overall; 0.91 in cohort studies). These are observational studies. People who eat yogurt may differ from those who do not in many ways, such as diet and exercise. We did not find a randomized trial of yogurt for colorectal cancer.

What official advice says

The NCCIH says: "If you're considering a probiotic dietary supplement, consult your health care provider first. This is especially important if you have health problems." It also says that in most cases, researchers still do not know which probiotics help, how much to take, or who benefits most.

The NCCIH explains that U.S. rules are complex. Many probiotics are sold as dietary supplements, which do not need FDA approval before sale. Supplement labels are not allowed to claim they lower your risk of a disease without FDA consent.

What about cancer?

The National Cancer Institute says that, with few exceptions, studies of human populations have not shown definitively that any dietary component causes or protects against cancer. We found no source we could open showing that a probiotic or a microbiome change treats cancer. Probiotics are not a cancer treatment.

For more on what helps with bowel side effects, see our pages on diarrhea during cancer treatment and when to call the doctor about chemo diarrhea. For fiber and bowel health, see fiber and colon cancer prevention. To learn about treatment types, see chemotherapy vs. immunotherapy.

What this story cannot tell you

  • It cannot tell you whether eating yogurt or taking a probiotic will change your own cancer risk. No randomized trial we opened tested that.
  • It cannot say whether a probiotic is safe for you during treatment. That depends on your treatment, your blood counts, whether you have a central line, and the product.
  • It cannot say that changing your gut bacteria will help a cancer treatment work. Much of that work is in mice or is an association.
  • We could not open the NCI PDQ summary on probiotics (the cancer.gov address returned "not found"), the American Cancer Society pages (blocked), or the FDA warning page on probiotics. We do not cite them. We also could not read the full text of the 2021 immunotherapy study, only its abstract, so our description of it is limited to that abstract.
  • Our search for harm in trials was not complete. We found no trial in which people on probiotics did significantly worse, but we did not review every trial.

Sources

How this article was prepared

An AI-assisted editorial system helped prepare this page. No named medical reviewer has reviewed it unless one is listed. We opened the sources above, except where a limit is stated in the text.

See an error, old source, or unclear wording? Tell us.

Put the story in context

Prevention, possible warning signs, screening, and diagnosis

This story relates to Diet and cancer. The information below is general, and not every point applies to every cancer. Personal advice depends on age, symptoms, family history, exposures, and medical history.

  • Prevention and risk reduction

    Not every cancer can be prevented. Avoiding tobacco, protecting skin from ultraviolet radiation, limiting alcohol, staying active, and receiving recommended HPV or hepatitis B vaccination can lower the risk of certain cancers. A risk factor is not a prediction or a cause in one individual.

    NCI prevention information ↗

  • Symptoms and possible early signs

    Possible signs vary and are often caused by conditions other than cancer. Changes worth discussing include a new lump, unexplained bleeding or weight loss, a persistent cough, lasting bowel or bladder changes, a changing skin spot, or symptoms that persist or worsen. Some early cancers cause no symptoms.

    NCI signs and symptoms ↗

  • Screening and early detection

    Screening looks for certain cancers before symptoms begin. Recommended tests exist only for some cancers and depend on age and risk. Screening can have benefits and harms; it is not the same as evaluating a new symptom, and there is no single routine scan or blood test that reliably screens for every cancer.

    NCI cancer screening information ↗

  • How cancer is diagnosed

    Diagnosis may involve a history and exam, imaging, laboratory tests, and often a biopsy. Pathology can identify the cancer type and may test biomarkers that guide treatment. Symptoms, screening results, tumor markers, or online stories alone cannot confirm cancer.

    NCI diagnosis information ↗

Learn more about diet and cancer

A news story may encourage questions, but it should not be used to estimate your risk or choose testing. Contact a healthcare professional about a persistent or concerning change. Seek urgent care for severe or rapidly worsening symptoms.

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