system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] Why do probiotics sometimes cause diarrhea, gas, or other symptoms of IBS?
gpt
Probiotics and antibiotics can both affect the amount of 'flora' (helpful bacteria) that live naturally inside your gut. Part of what those bacteria do is help digest things and create important chemicals that the body uses. Bacteria can act almost act like an organ in our body. Cows, for example need gut bacteria to be able to eat grass. We have different bacteria than cows, so we don't get the luxury of eating grass as food.

Gut bacteria unsurprisingly have some interaction with our immune system, whose favorite pastime is inflammation. Changes in flora can coincide with changes in inflammation, such that flora can ultimately decrease gut inflammation. But since the body keeps things in a delicate balance, periods of transition can be irritating.

Regarding gas, bacteria utilize a process called fermentation, which chemically results in gas bi-products (carbon dioxide, methane).

Ultimately, increasing or decreasing the amount and/or balance of natural bacteria in the gut can cause other changes in response while a balance is being achieved.