Some probiotics appear to be as effective as the drug mesalazine in the treatment of ulcerative colitis. A variety of preparations have shown effects of preventing relapse or maintaining remission. 

Does it work?

Some probiotics appear to be as effective as the drug mesalazine in the treatment of ulcerative colitis. A variety of preparations have shown effects of preventing relapse or maintaining remission. A probiotic combination consisting of probiotics plus balsalazide may be more effective than balsalazide or mesalazine alone. More studies are needed to more clearly determine what outcomes can be expected.

Probiotics show promise for preventing atopic dermatitis (eczema) in children. Infants benefit when their mothers take probiotics during pregnancy and breastfeeding. Direct supplementation of infants may reduce the incidence of atopic eczema by as much as half. It may also reduce cow's milk allergy and other allergic reactions during weaning. Probiotics may stabilize intestinal barrier functioning and decrease gastrointestinal symptoms in children with atopic dermatitis.

Many varieties and combinations of probiotics have been studied in clinical trials for irritable bowel syndrome (IBS). Findings frequently report reductions of symptoms including pain, flatulence, bloating, and stool frequency. There is some evidence of reduced inflammation.
 

Dangers and possible side effects....

Probiotics are generally regarded as safe for human consumption. Long-term consumption of probiotics is considered safe and well tolerated. Avoid if allergic or hypersensitive to probiotics. Use cautiously if lactose intolerant. Caution is advised when using probiotics in neonates born prematurely or with immune deficiency.

References:

https://en.wikipedia.org/wiki/Probiotic

http://www.medicinenet.com/probiotics/article.htm