Both probiotics & antibiotics have their purposes, but they butt heads when used together. I'm so glad you asked about that, actually, because when one person asks, there's very likely a bunch of others that aren't posting, only reading along, that might be confused about the very same thing. We often talk about probiotics as being essentially harmless but if used improperly (at the same time as necessary antibiotic therapy) they can do bad things (like impair the needed medication).