Probiotic supplements, often sold as dietary aids, are marketed to improve a variety of health conditions, including digestive issues, mental health, and hormonal balance. However, recent research suggests that for certain groups, such as cancer patients and those with weakened immune systems, these supplements might be more harmful than beneficial. While probiotics are generally considered safe for healthy individuals, their effectiveness can be limited due to the complex nature of the human microbiome — the community of microorganisms living in the body, primarily in the gut. Suzanne Devkota, director of the Human Microbiome Research Institute at Cedars Sinai, once thought over-the-counter probiotics were at most a waste of time for healthy people. This view still holds, as the natural microbiome can often overshadow the probiotic, making them ineffective but not harmful. However, two articles published in 2018 shifted the scientific understanding of probiotics. These studies showed that in situations where probiotics were traditionally thought to be helpful, such as after antibiotic use, they could actually be harmful for immunocompromised individuals or cancer patients undergoing treatment. The challenge lies in applying a one-size-fits-all approach to a system as complex as the microbiome. The ideal mix of microbes for a healthy young person may not be the same as for someone with a chronic illness. Although it is theoretically possible to analyze one’s microbiome and choose a probiotic that fills specific gaps, current commercial products are not yet able to achieve this level of personalization. A study involving 21 participants found that those who took commercial probiotics had the slowest recovery of their gut microbiomes after antibiotic treatment, while those who received fecal transplants recovered much more quickly. Eran Elinav, an immunologist at the Weizmann Institute of Science in Israel, explains that commercial probiotics can be counterproductive in the context of recovering from antibiotic use. The limited number of bacterial strains in these products can interfere with the natural diversity of gut bacteria, which typically contains thousands of different strains. This effect can vary widely between individuals. These findings are especially relevant for cancer patients undergoing immunotherapy, as studies have shown that a diverse microbiome can improve treatment outcomes. However, some probiotics may interfere with the benefits of a high-fiber diet, which is often recommended for these patients. The role of the microbiome during pregnancy remains unclear. Some research suggests that probiotics might help prevent conditions like gestational diabetes or preeclampsia, but meta-analyses have not confirmed these benefits. In fact, some studies indicate a possible increased risk of preeclampsia. Due to the lack of strict regulation, the quality of over-the-counter probiotics can vary, and it is generally advised to avoid them during pregnancy. In specific situations, such as treating infectious diarrhea, probiotics can be helpful. However, in many cases, the benefits of commercial probiotics do not live up to their marketing claims. Experts recommend a varied diet rich in fiber and natural sources of probiotics, such as yogurt and kombucha, even though their effects are temporary. While future advancements may lead to more effective probiotics, as suggested by Jennifer Wargo, a professor of surgical oncology at the MD Anderson Cancer Center, for now, caution is advised, especially for those in vulnerable health conditions.