It may be safe to use probiotics along with a fiber supplement like Metamucil, but the effectiveness of probiotics for IBS is not definitive. Some people may experience diarrhea as a side effect of probiotics, so it is important to be cautious. Make sure to space out taking Metamucil and any other medications by at least 3 hours to avoid any interactions. Ultimately, it is up to you to decide if you want to incorporate probiotics into your treatment plan.