Probiotics can help restore the balance of gut bacteria. Studies on the effectiveness of probiotics have been mixed, and it is unclear how probiotics can improve health.
Probiotics are often taken:
- Along with antibiotics, to prevent diarrhea caused by the antibiotics
- To reduce gas, bloating, and constipation
Studies are still being done to investigate if probiotics may help:
- Fight or prevent infections in the stomach or intestines, including C. difficile (C. diff) diarrhea
- Treat diarrhea, constipation, and conditions that may cause these symptoms, such as irritable bowel syndrome (IBS) and inflammatory bowel diseases (IBD) such as ulcerative colitis and Crohn's disease
- Prevent or treat allergies
- Prevent or treat eczema
- Prevent or fight vaginal infections
What Are Probiotics?
Probiotics are what is often referred to as “good bacteria” or “friendly bacteria.” Not all bacteria cause illness. Probiotics are a type of bacteria that live in the body and help keep the gut healthy and can help defend the body from infections.
Probiotics and prebiotics are not the same. Prebiotics are a source of food for probiotics.
Do You Need to Take Probiotic Supplements?
If you eat a healthy diet comprised of fruits and vegetables, whole grains, beans and legumes, yogurt, and fermented foods such as kimchi, kefir, and sauerkraut you should be able to maintain a healthy gut without probiotic supplements.
Yogurt products labeled as having “active cultures” contain probiotics. This may or may not help with digestion. If you enjoy yogurt and have no problems digesting it, it is fine to include as part of a healthy, balanced diet.
What Are the Risks of Taking Probiotics?
In many cases, even if probiotics do not help, they are not necessarily harmful. However, there may be some risks with probiotic supplements:
- Probiotics are considered dietary supplements and are not regulated by the Food and Drug Administration (FDA) like medications, which means products may not contain what the labels claim they do.
- People with weakened immune systems (such as people on cancer chemotherapy or with certain illnesses) may get an infection following the use of probiotics.
Talk to your doctor before adding probiotic supplements to your diet.
From 
https://www.uptodate.com/contents/probiotics-the-basics?search=probiotics&source=search_result&selectedTitle=1~135&usage_type=default&display_rank=1