@hopeful33250 a prescription is not necessary, but it is a product that isrecommended by gastrointestinal physician. Their is a website vsl.com and the VSL#3 is what I use. It is 450 billion units. They do have one that is 900 billion units but it is necessary to get a prescription for that. Check out their website, ( https://www.vsl3.com) they do have a lot of information on it and they do offer discounts for subscriptions and new subscribers. Hope this helps!