Probiotics

Probiotics are supplements containing bacteria. Research has found probiotics good for certain conditions (such as antibiotic-induced diarrhea), but woo-meisters frequently make extravagant and unsubstantiated claims about probiotics' ability to "boost the immune system" and "replenish the gut flora" and the like. A 2018 study concluded that probiotics were largely useless.

Since probiotics, like many other nutritional supplements, are practically unregulated, the actual contents of the pills can differ substantially from what's on the label. One study has shown that probiotics often contain bacteria not listed on the label and do not contain species that are listed on the label.

Prebiotics
Prebiotics are nutrients selectively used by beneficial microbes that occur naturally in the gut, such as Lactobacillus and Bifidobacteria. Unlike probiotics, prebiotics do not need to contain live bacteria, making them easier to store and formulate. Instead, they are usually "fermentable fibers", i.e. particular oligosaccharides or polysaccharides, which are indigestible to humans, but digestible to these bacteria. Natural sources include for example onion, wheat, barley and asparagus. However, they may be also added to foods. While there is some promising preliminary research on their effectiveness for some conditions, hold your horses: clinical evidence is still scarce and derived from a small number of patients. This applies to inflammatory bowel disease, prevention of colon cancer, reduction of infections in infants, and hypertension. Results for irritable bowel syndrome and prevention of infections in adults are unconvincing, and prebiotics have no effect on blood glucose levels in diabetics or on cholesterol. Nevertheless, while there is some science behind this, adding prebiotics to food products and selling them as "health foods" would be very easy and cheap, and would make a good business.