*2.1. Design*

A systematic review of randomized controlled trials was undertaken in January 2021, following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Supplementary File 1) [22]. This review used a structured Patient– Intervention–Outcome (PIO) question as follows [23]: "In adult oncology patients (P), what is the efficacy of probiotics supplements (I) on treatment-related side effects (O)?" The protocol for this review was not registered.

## *2.2. Search Strategy*

The electronic databases included PubMed, Scielo, ProQuest, and OVID, using natural and structured language in the following search strategy: (((((probiotics [Title/Abstract] OR probiotics [MeSH Terms]) OR lactobacillus [Title/Abstract]) OR bifidobacterium [Title/Abstract]) OR lactobacillus [MeSH Terms]) OR bifidobacterium [MeSH Terms]) AND ((((radiotherapy [Title/Abstract] OR chemotherapy [Title/Abstract]) OR chemotherapy [MeSH Terms]) OR radiotherapy [MeSH Terms]) OR radiation [Title/Abstract]). This search strategy was adapted for use across databases (Appendix A). "Randomized clinical trial", "humans", and "adult:19+ years" search filters were applied for this search strategy.

## *2.3. Selection Criteria*

The following inclusion criteria were used: (i) randomized clinical trials, (ii) published in English or Spanish, (iii) related to the aim of the study; the use of probiotics supplements on adult oncology-related treatments and side effects; and (iv) published until January 2021. Likewise, the exclusion criteria included (i) studies on other pathologies than cancer or symptoms related to cancer treatments, (ii) symbiotics and other treatment combinations, (iii) re-publications, and (iv) studies with animals. No articles were excluded after quality appraisal.
