Dosages vary from one product to the next, so no general, overarching recommendation can be made. However, a common dosage for an adult or senior ranges from 5 billion to 10 billion colony-forming units per day. Again, this may vary depending upon the probiotic you choose. Probiotics should be taken in one dose every 24 hours, just be wary that some people may experience gas or loose stools in the first few days of taking them.