Probiotics During Cancer Treatment: Radiation, Chemotherapy and Why the Oncology Team Must Be Asked

Ask your cancer team first

During cancer treatment, do not start, stop or swap any probiotic or other supplement without asking your oncology team or pharmacist, including probiotic foods and drinks. Your team will have told you who to contact for fever, severe diarrhea, vomiting or signs of dehydration; use that contact, or emergency care (911 in the US), rather than treating it yourself with a supplement. Nothing here is a reason to skip or change cancer treatment. See integrative care during cancer treatment: what to tell your oncology team.

The short answer

Reviews find that probiotics may lower diarrhea from chemotherapy or radiotherapy, but the evidence is low-certainty and made up of small, varied trials. Case reports show that probiotic organisms can, rarely, cause bloodstream infections in people with cancer, and an observational study linked probiotic supplements to poorer responses to immunotherapy in melanoma. So this is a decision for the oncology team, case by case. NCCIH advises that anyone with a serious underlying condition be monitored closely, and to consult a provider before taking a probiotic. [1]

Evidence labels: promising but limited, mixed, insufficient evidence, evidence of harm (case reports), preclinical. Study amounts are not advice.

Diarrhea from radiotherapy or chemotherapy

Label: promising but limited. The Cochrane authors noted that treatment-related diarrhea is linked to a higher risk of infection in people with neutropenia (a treatment side effect your team will explain) and often leads to treatment delays or dose reductions, which is why the question matters. [2] They also called for better-designed trials. [2]

What supportive-care guidelines say

The Multinational Association of Supportive Care in Cancer and International Society of Oral Oncology (MASCC/ISOO) reviewed agents for gut inflammation caused by cancer treatment. Its 2013 panel suggested that probiotics containing Lactobacillus species may be beneficial for preventing chemotherapy- and radiotherapy-induced diarrhea in people with pelvic cancers. [4] Its 2019 update made no new guidelines because of inadequate or conflicting evidence and said the evidence continued to support Lactobacillus probiotics for prevention in pelvic malignancy. [5] That is a guideline from a specialist group about a specific setting, not a general recommendation for every person on treatment. A separate 2020 MASCC/ISOO review of oral mucositis included probiotics in its scope; its new suggestions concerned honey and chewing gum, and it said no guideline was possible for other interventions. [6]

Mouth sores (oral mucositis)

Meta-analyses report fewer cases of severe mouth sores with probiotics (a 2022 review of 8 trials and 708 patients; a 2024 review of 12 articles and 1,055 patients). [7][8] The 2024 review found no significant difference in diarrhea. [8] Each review pools few trials, and the 2020 MASCC/ISOO update above made no probiotic guideline. Label: mixed.

The infection concern

• A 2018 review of 25 safety studies (2,242 people) found five case reports of probiotic-related bloodstream infection (bacteria or fungi in the blood), and said adverse event reporting was variable and inconsistent. [9] A 2014 review also found five such case reports and concluded probiotics may be a rare cause of sepsis. [10]
• A 2024 case report describes a 75-year-old woman with lymphoma who developed febrile neutropenia during chemotherapy and a Bifidobacterium breve bloodstream infection; her usual yogurt did not contain that species. [11] The authors speculated that fermenting bacteria could be a source of infection in people on chemotherapy. [11] A single case cannot show cause.
• NCCIH says the risk of harm is greater with severe illness or weakened immunity. [1] See probiotics and bloodstream infection and probiotic safety.

Immunotherapy

A study in 128 patients with advanced melanoma treated with immune checkpoint inhibitors found better progression-free survival with higher dietary fiber, most clearly in people who also did not use probiotic supplements; mice given probiotics also responded worse. [12] The NCI notes that nearly a third of patients reported recent probiotic use and that the small sample and variety of products made firm conclusions difficult. [13] Label: preclinical plus observational; not proof of cause. Ask your team before combining immunotherapy with a probiotic.