Should Cancer Patients Take Probiotics During Immunotherapy?

Research-informed explainer — last updated April 12, 2026

Probiotics are widely considered safe and beneficial. But for cancer patients receiving checkpoint inhibitor immunotherapy — drugs like pembrolizumab (Keytruda) or nivolumab (Opdivo) — the picture is more complicated. A growing body of research suggests that the specific composition of your gut bacteria directly predicts whether immunotherapy works, and that commercial probiotic supplements may actually shift that composition in the wrong direction.

This explainer draws on research from three oncologists in the Convene directory. Thomas Gajewski at the University of Chicago is the researcher whose landmark 2015 and 2018 studies in Science first established the microbiome-immunotherapy connection in both animal models and human melanoma patients. Michael Kim at MD Anderson Cancer Center contributed to research showing that microbiome composition within the tumor itself predicts outcomes in pancreatic cancer. Chad Michener at Cleveland Clinic co-authored research demonstrating that antibiotic-induced gut microbiome disruption directly confers treatment resistance in ovarian cancer.

How does the gut microbiome affect immunotherapy?

Checkpoint inhibitors work by releasing a brake on your immune system. Drugs that block PD-1 or PD-L1 — proteins that ordinarily tell T cells to stand down — allow those T cells to recognize and attack cancer cells. The drugs work remarkably well for some patients and not at all for others, and oncologists have spent years trying to understand why.

Gajewski's 2015 study in Science provided a striking answer: the bacteria living in your gut are part of the explanation [1]. Working with mouse models of melanoma, his group found that mice with higher levels of Bifidobacterium in their gut had substantially better spontaneous antitumor immune responses. More importantly, when those mice received anti-PD-L1 therapy, the combination produced near-complete tumor control. Mice without Bifidobacterium had weak immune responses that anti-PD-L1 alone could not rescue. The mechanism runs through CD8+ T cell function — the gut bacteria were directly influencing the activation and quality of the immune cells responsible for killing tumors.

When the researchers transferred Bifidobacterium into mice that lacked it, antitumor immunity improved. The bacteria were not bystanders; they were active participants in how well the immune system fought cancer.

What did the human data show?

The 2015 findings were in mice. The 2018 follow-up study — also from Gajewski's group, published in Science — tested whether the same principle applied to people [2].

The researchers analyzed the gut microbiome composition of patients with metastatic melanoma who were receiving anti-PD-1 therapy. The results closely mirrored the mouse data. Patients whose gut microbiome was enriched with certain bacterial species — including Faecalibacterium prausnitzii and members of the Ruminococcaceae family — had significantly higher rates of response to treatment, longer progression-free survival, and stronger CD8+ T cell infiltration into their tumors. Patients with less favorable microbiome profiles had lower response rates and weaker immune activation.

Critically, the gut bacteria were predictive even before treatment started. The microbiome composition at baseline — before the first dose of immunotherapy — was already associated with whether the drug would work.

Where do probiotics fit in, and why might they cause problems?

Most commercial probiotic supplements are heavily dominated by Lactobacillus and Bifidobacterium strains. At first glance, Bifidobacterium sounds like it should be beneficial — the same genus that appeared in Gajewski's mouse research. But there are important distinctions.

First, commercial probiotic Bifidobacterium strains are not necessarily the same strains associated with immunotherapy response. Second, and more concerning, several research groups have reported that Lactobacillus-dominant gut profiles — the kind that probiotic supplements reliably produce — appear to be associated with reduced immunotherapy response rather than enhanced response. The favorable microbiome signature associated with checkpoint inhibitor response tends to be dominated by different species altogether, particularly Faecalibacterium, Ruminococcus, and Akkermansia families.

The deeper problem is that probiotic supplements do not simply add bacteria — they can alter the overall composition of your gut microbiome, potentially crowding out or suppressing the bacterial communities that actually support immune function. Introducing large doses of Lactobacillus species may shift the ecological balance away from the specific bacterial profile that checkpoint inhibitors need to work effectively.

This is not settled science. But the theoretical concern is real enough that several oncologists now advise patients on immunotherapy to avoid probiotic supplements, at least until more data are available.

What happens when the gut microbiome is disrupted entirely?

Research from Michener's group at Cleveland Clinic adds a different angle on the same principle [6]. Their 2022 study in Cancer Research examined what happens to treatment response when the gut microbiome is severely disrupted — specifically in the context of epithelial ovarian cancer treated with cisplatin.

The finding was striking: disruption of the gut microbiota conferred cisplatin resistance. Patients whose gut microbiome was depleted — as happens with broad-spectrum antibiotics — were less able to respond to chemotherapy. The gut bacteria were not merely correlating with outcomes; removing them changed the biology of treatment resistance.

This connects to an increasingly recognized pattern in oncology: antibiotic use in the weeks before or during immunotherapy is associated with worse outcomes. Antibiotics wipe out the gut microbiome non-selectively, potentially eliminating the exact bacterial communities that support immune activation. While the antibiotic-immunotherapy interaction is more studied and more clearly established than the probiotic-immunotherapy question, both point in the same direction: the gut microbiome is not incidental to cancer treatment.

Is this limited to skin cancer and ovarian cancer?

The microbiome-cancer connection extends well beyond melanoma. Michael Kim's research at MD Anderson, published in Cell in 2019, examined the tumor microbiome — the bacteria living directly within pancreatic tumors — and found that microbial diversity and composition influenced survival outcomes [5].

Patients with higher intratumoral microbial diversity had longer survival. When the researchers transferred gut bacteria from long-term pancreatic cancer survivors into germ-free mouse models, those mice showed enhanced tumor control. The reverse transfer — from non-survivors — produced the opposite effect. The bacteria within the tumor were not random contamination; they were shaping the tumor immune microenvironment in ways that predicted whether a patient lived longer.

Taken together, the gut microbiome data across melanoma, ovarian cancer, and pancreatic cancer tell a consistent story: bacterial communities in and around tumors influence how the immune system responds to cancer and to cancer treatment.

What is being tested right now?

If a favorable microbiome composition predicts immunotherapy response, the obvious question is whether you can give patients that composition. Fecal microbiota transplantation (FMT) trials are currently underway at several cancer centers to test exactly that hypothesis. In FMT, stool from a carefully selected donor — ideally a long-term immunotherapy responder — is transferred to a patient to repopulate their gut microbiome.

Early results from small studies have been encouraging: a handful of patients with melanoma who had stopped responding to anti-PD-1 therapy appeared to regain responses after receiving FMT from immunotherapy responders. These are preliminary findings from very small cohorts, not controlled trials. But they suggest the causal direction is real — that altering the microbiome can change immunotherapy outcomes — and have justified the larger trials now enrolling patients.

FMT is not currently a standard treatment. It remains investigational for this indication. But the fact that major cancer centers are running these trials reflects how seriously the field takes the microbiome-immunotherapy connection.

Questions to ask your oncologist

• Should I avoid probiotic supplements during my checkpoint inhibitor therapy, and if so, for how long?
• Have you considered my gut microbiome composition as part of my treatment plan?
• Has my recent antibiotic use — within the past few months — affected my likelihood of responding to immunotherapy?
• Am I eligible for any clinical trials that involve microbiome modification or FMT as part of immunotherapy?
• If I develop an infection that requires antibiotics during treatment, is there a strategy to support microbiome recovery afterward?

The bottom line

The gut microbiome is not a side story in cancer immunotherapy — it is directly connected to whether checkpoint inhibitors work. Gajewski's foundational research established that specific gut bacteria augment antitumor T cell function and enhance anti-PD-L1 efficacy, and that patients with favorable microbiome profiles have meaningfully higher response rates. Research from Michener's group showed that microbiome disruption can confer treatment resistance outright. Kim's work at MD Anderson extends the finding to the tumor microbiome itself in pancreatic cancer.

Commercial probiotic supplements are not yet proven to harm immunotherapy outcomes — but the mechanism by which they could is biologically plausible and increasingly taken seriously by oncologists. The strains in most supplements do not match the bacterial profile associated with immunotherapy response, and supplementation may shift gut composition in the wrong direction. Until larger controlled studies resolve the question, self-supplementing with probiotics during checkpoint inhibitor therapy is worth discussing with your oncologist rather than doing on your own.

Antibiotic avoidance during immunotherapy is more clearly established and more widely recommended. The broader principle — that your gut microbiome is an active variable in treatment response, not a background condition — is now supported by thousands of citations across multiple cancer types and multiple treatment modalities.