If you have an immune system problem or another serious health condition, you may have a greater chance of issues. Some reports have linked probiotics to serious infections and other side effects. The people most likely to have trouble are those with immune system problems, people who've had surgery, and others who are critically ill. Don't take probiotics if you have any of those issues.
"People have thrown a lot of support to probiotics, even though the literature underlying our understanding of them is very controversial; we wanted to determine whether probiotics such as the ones you buy in the supermarket do colonize the gastrointestinal tract like they're supposed to, and then whether these probiotics are having any impact on the human host," says senior author Eran Elinav, an immunologist at the Weizmann Institute of Science in Israel. "Surprisingly, we saw that many healthy volunteers were actually resistant in that the probiotics couldn't colonize their GI tracts. This suggests that probiotics should not be universally given as a 'one-size-fits-all' supplement. Instead, they could be tailored to the needs of each individual."
While Lebwohl isn't comfortable endorsing a specific brand publicly, he did say "a widely used brand in which the compound is bifidobacteria or lactobaccilus seems like a reasonable way to go in a patient who is eager to give this a try, but would be best done under the supervision of a healthcare professional.” He does advise looking for a supplement with between 1 billion and 10 billion colony-forming units, or CFUs.

^ Free shipping on purchases of 49 USD or more calculated after any coupon codes are applied and before any applicable sales tax and shipping charges are applied; valid only within the 48 contiguous U.S. states. Orders less than 49 USD will be charged 3.95 USD shipping. A surcharge of 4.99 USD will be applied to shipments to AK, AS, FM, GU, HI, MH, MP, PR, and VI. International shipping costs vary depending on weight and destination. Additional charges may apply on orders over $1,000 USD.
A healthy digestive system is vital to good health. The food that you eat is processed and the nutrients you consume are absorbed in the digestive system. However, it is also home to much of your immune system. This makes it important for helping you fight off illnesses. Probiotic supplements may help to balance the gut to promote increased digestive health and improved overall health as a result. However, more studies are needed.
^ Jump up to: a b c Rijkers GT, Bengmark S, Enck P, Haller D, Herz U, Kalliomaki M, Kudo S, Lenoir-Wijnkoop I, Mercenier A, Myllyluoma E, Rabot S, Rafter J, Szajewska H, Watzl B, Wells J, Wolvers D, Antoine JM (2010). "Guidance for substantiating the evidence for beneficial effects of probiotics: current status and recommendations for future research". J. Nutr. 140 (3): 671S–6S. doi:10.3945/jn.109.113779. PMID 20130080.
Probiotics act by stimulating the growth of microorganism colonies in our bodies that are “good” or helpful. These beneficial bacteria play an important role in maintaining the natural balance in our systems, stabilizing our digestive organs’ barriers against undesirable microorganisms, producing substances that inhibit “bad” microorganisms’ growth, outcompeting undesirable microorganisms, and stimulating immune responses.1 

Inflammatory bowel disease (IBD) treatment with antibiotics is fraught by variable efficacy. Recent discovery of genetic polymorphisms (NOD2/CARD15) in CD which play a role in bacterial peptidoglycan recognition [Inohara et al. 2003; Kobayashi et al. 2005], may be responsive to alterations in enteric flora and thus be important in the pathogenesis and maintenance of IBD [Sartor, 2004].
Taking probiotics can also help keep your urinary system working properly. While you might think that yeast imbalances or urinary tract issues are primarily women's problems, both are common in men as well, especially in those whose bacterial balance is off-kilter. Taking probiotics can help address these issues by encouraging the growth of good bacteria to crowd out unwanted yeast.10

“Microbiome” refers to the trillions of bacteria and microorganisms that live on and in our individual bodies. “There’s increased recognition of the various ways that the microbiome affects our health,” Lebwohl tells me. “And with that recognition comes the true observation that many bacteria are good for us. This goes against the old idea that bacteria equal germs, which equal harm. It turns out that many bacteria keep us healthy and probiotics could potentially support that notion.”
Research suggests probiotics work better as a team. Even pairs are more effective than individuals. Take the strains Lactobacillus Acidophilus and Bifidobacterium Bifidum BB-12, for example. When combined, this duo-force has been shown to help treat GI-specific ailments. In fact, one survey found 75 percent of studies that compared the effect of a strain mixture with a single-strain supplement showed a mixture was more effective at improving irritable bowel symptoms, immune function, and digestive health.
If you struggle with anxiety... Your brain and gut talk on the reg, and both produce neurotransmitters such as serotonin (the happiness chemical). Experts think balancing your guy may help treat anxiety, because it promotes better communications between the two. Opt for lactobacillus plantarum and bifidobacterium bi-fi0dum, two strains for anxiety, per research. Try 10 billion CFUs for three to five days. If your tum feels fine, go up by 10 bill.

Myriad factors – antibiotics, diet, stress, and age, among them – affect the balance of diverse microbes present in your gut. While you can replenish your gut bacteria by eating well and incorporating natural probiotics (ex. yogurt and kefir) into a healthy diet, these processes can take weeks or months; taking a regular probiotic is an easy and effective way to ensure your gut (and immune system) stays healthy, always.

Probiotics are live bacteria that are present in yogurt, other fermented foods, and in pills. They are promoted as a benefit to the human digestive system. Normally you have trillions upon trillions of bacteria within the colon. Normally we ingest bacteria every time we swallow. Many of these swallowed bacteria may be beneficial while most are simply innocuous and cause no problems. The question for everyone who takes a probiotic much beyond yogurt is whether they really are a health benefit. Here is what we know medically about probiotics.

When your gut is healthy, you have a large, thriving population of beneficial or friendly bacteria, or probiotics, supporting your immune system receptor cells. They help form a protective barrier within your colon and intestines. Optimizing and supporting the beneficial bacteria in your gut is one of the most powerful things you can do for your health and well being, including your immune health.
Since the effects of individual bacteria strains vary, the first thing to consider when choosing a probiotic supplement is the reason you are taking it. Certain strains, for example, may help with weight loss, lower cholesterol or reduce allergy symptoms, while others have been shown to help with digestive issues, such as diarrhea from antibiotics and irritable bowel syndrome. The uses and evidence for various strains are summarized in a reference table in the Probiotic Supplements Review (be aware that certain strains should be avoided by people with milk allergies or people taking certain medications).

Digestive problems. The best evidence for probiotics is for reducing diarrhea, especially following antibiotic use. A 2010 review from the Cochrane Collaboration concluded that probiotics shorten episodes of acute infectious diarrhea. And in 2011, a Health Canada monograph stated that products containing certain probiotics (such as Lactobacillus rhamnosus GG) help manage acute infectious diarrhea and antibiotic-associated diarrhea. A 2012 research review in the Journal of the American Medical Association (JAMA) found that probiotics reduced the risk of antibiotic-associated diarrhea by 42 percent—but many of the studies had flaws, so these findings should be interpreted with caution. A 2013 Cochrane review of 23 trials also concluded that probiotics may be effective for preventing antibiotic-related diarrhea. However, two large, well-designed studies, in the Lancet in 2013 and the Annals of Internal Medicine in 2014, found that probiotics were no better than a placebo in preventing diarrhea in older people taking antibiotics. A review of 19 studies, published in Gastroenterology in 2017, found that probiotics reduce the risk of Clostridium difficile-related diarrhea in hospital patients, especially when the supplements were started during the first two days of antibiotic treatment.
Several large-scale studies and two meta-analyses have confirmed that probiotics should be a major consideration in determining natural remedies for diabetes. In a massive study involving almost 200,000 subjects and a total of 15,156 cases of type 2 diabetes, researchers confirmed that a higher intake of probiotic-rich yogurt reduced the risk of developing diabetes.

If you have annoying tummy troubles... Allow your predominant symptoms to guide you. Bloated and constipated? Look for lactobacillus acidophilus or lactobacillus casei, which help restore gut flora. Can't stop going number two? Types with saccharomyces boulardii can ease antibiotic-associated diarrhea and even prevent extreme cases caused by infections.
Jotham Suez, Niv Zmora, Gili Zilberman-Schapira, Uria Mor, Mally Dori-Bachash, Stavros Bashiardes, Maya Zur, Dana Regev-Lehavi, Rotem Ben-Zeev Brik, Sara Federici, Max Horn, Yotam Cohen, Andreas E. Moor, David Zeevi, Tal Korem, Eran Kotler, Alon Harmelin, Shalev Itzkovitz, Nitsan Maharshak, Oren Shibolet, Meirav Pevsner-Fischer, Hagit Shapiro, Itai Sharon, Zamir Halpern, Eran Segal, Eran Elinav. Post-Antibiotic Gut Mucosal Microbiome Reconstitution Is Impaired by Probiotics and Improved by Autologous FMT. Cell, 2018; 174 (6): 1406 DOI: 10.1016/j.cell.2018.08.047