Do probiotics make you poop?
Maybe, slowly, and only some kinds. Probiotics aren't a laxative. Some strains sped things up a little in trials, over weeks, while others did nothing. Gut doctors don't routinely recommend them for constipation.
Did probiotics make you go?
The meter says what should happen. Your bowels get a vote too.
Why it works
A small effect, on average
Pooling 14 trials of 1,182 adults, probiotics cut gut transit time by about 12 hours and added roughly one bowel movement a week. The researchers rated many of the trials as weak.
The strain matters
Bifidobacterium lactis strains helped the most. Other strains in the same review didn't improve anything, so "probiotics" as a whole isn't one answer.
Mostly safe, with one big exception
The usual side effects of probiotics are mild gut ones like gas. People with weakened immune systems are at risk of more serious problems and should ask first.
Sources: Dimidi et al., Am J Clin Nutr 2014; NCCIH, Probiotics: usefulness and safety.