Abstract: We included 12 studies involving 1554 participants. Eleven studies were prevention studies, of which seven compared probiotics with placebo (887 participants), one compared two doses of probiotics with each other and with placebo (246 participants), and three compared probiotics with another active agent (216 participants).The remaining study assessed the effectiveness of probiotics compared with placebo for treatment of radiotherapy-related diarrhoea (205 participants). For prevention of radiotherapy (with or without chemotherapy)-induced diarrhoea, review authors identified five heterogeneous placebo-controlled studies (with 926 participants analysed). Owing to heterogeneity, we could not carry out a meta-analysis, except for two outcomes. For occurrence of any diarrhoea, risk ratios (RRs) ranged from 0.35 (95% confidence interval (CI) 0.26 to 0.47) to 1.0 (95% CI 0.94 to 1.06) (three studies; low-certainty evidence). A beneficial effect of probiotics on quality of life could neither be demonstrated nor refuted (two studies; low-certainty evidence). For occurrence of grade 2 or higher diarrhoea, the pooled RR was 0.75 (95% CI 0.55 to 1.03; four studies; 420 participants; low-certainty evidence), and for grade 3 or higher diarrhoea, RRs ranged from 0.11 (95% CI 0.06 to 0.23) to 1.24 (95% CI 0.74 to 2.08) (three studies; low-certainty evidence). For probiotic users, time to rescue medication was 36 hours longer in one study (95% CI 34.7 to 37.3), but another study reported no difference (moderate-certainty evidence). For the need for rescue medication, the pooled RR was 0.50 (95% CI 0.15 to 1.66; three studies; 194 participants; very low-certainty evidence). No study reported major differences between groups with respect to adverse effects. Although not mentioned explicitly, no studies reported deaths, except one in which one participant in the probiotics group died of myocardial infarction after three sessions of radiotherapy. Three placebo-controlled studies, with 128 analysed participants, addressed prevention of chemotherapy-induced diarrhoea. For occurrence of any diarrhoea, the pooled RR was 0.59 (95% CI 0.36 to 0.96; two studies; 106 participants; low-certainty evidence). For all other outcomes, a beneficial effect of probiotics could be neither demonstrated nor refuted (one to two studies; 46 to 106 participants; all low-certainty evidence). Studies did not address quality of life nor time to rescue medication. Three studies compared probiotics with another intervention in 213 participants treated with radiotherapy (with or without chemotherapy). One very small study (21 participants) reported less diarrhoea six weeks after treatment when dietary counselling was provided (RR 0.30, 95% CI 0.11 to 0.81; very low-certainty evidence). In another study (148 participants), grade 3 or 4 diarrhoea occurred less often in the probiotics group than in the control group (guar gum containing nutritional supplement) (odds ratio (OR) 0.38, 95% CI 0.16 to 0.89; low-certainty evidence), and two studies (63 participants) found less need for rescue medication of probiotics versus another active treatment (RR 0.44, 95% CI 0.22 to 0.86; very low-certainty evidence). Studies did not address quality of life nor time to rescue medication. One placebo-controlled study with 205 participants addressed treatment for radiotherapy-induced diarrhoea and could not demonstrate or refute a beneficial effect of probiotics on average diarrhoea grade, time to rescue medication for diarrhoea (13 hours longer in the probiotics group; 95% CI -0.9 to 26.9 hours), or need for rescue medication (RR 0.74, 95% CI 0.53 to 1.03; moderate-certainty evidence). This study did not address quality of life. No studies reported serious adverse events or diarrhoea-related deaths. This review presents limited low- or very low-certainty evidence supporting the effects of probiotics for prevention and treatment of diarrhoea related to radiotherapy (with or without chemotherapy) or chemotherapy alone, need for rescue medication, or occurrence of adverse events. All studies were underpowered and heterogeneous. Severe side effects were absent from all studies. Robust evidence on this topic must be provided by future methodologically well-designed trials.

Plain-language summary: Overall, the studies we found do not give a clear answer on whether probiotics reduce the occurrence or severity of diarrhoea, improve quality of life, or reduce the need for other medication. However, an analysis of only well-performed studies demonstrated a beneficial effect for some outcomes. With regard to prevention of diarrhoea compared with placebo in participants treated with radiotherapy with or without chemotherapy, we are not able to conclude whether use of probiotics would be beneficial based on the five relevant studies. For prevention of diarrhoea due to chemotherapy alone, three studies suggested that use of probiotics may not reduce diarrhoea, and one study reported use of less rescue medication for diarrhoea. Three studies that compared probiotics with another agent for preventing diarrhoea in patients treated with radiotherapy with or without chemotherapy found beneficial effects of probiotics for the occurrence and severity of diarrhoea and the need for rescue medication. With respect to treatment of diarrhoea due to radiotherapy, we found only one study that did not demonstrate a clear effect of probiotics compared with placebo. No study reported serious adverse events nor deaths related to diarrhoea. The quality (certainty) of the evidence in prevention studies was low to very low. For the only study that assessed the effects of probiotics on treatment for diarrhoea, the certainty of the evidence was moderate. Evidence supporting the effects of probiotics in preventing or treating diarrhoea related to cancer treatment is insufficient. However, probiotics appear to be safe, as no studies have found severe side effects.