There is absolutely no harm in taking 'One week flat' on a daily basis and for a long period of time.
OptiBac 'One week flat' is a 7 day intensive course of live cultures & FOS fibres in powdered form. A daily dose contains 5 billion microorganisms including Lactobacillus acidophilus Rosell-52, as well as 0.3mg of FOS. Most customers find the effects of a 7 day consecutive course to last for another 2-3 weeks. For this reason, many people take this product on a monthly basis, either taking no supplements in between or taking OptiBac 'For every day' or 'For every day EXTRA Strength' during the other 3 weeks.
However, if you find that 'One week flat' works best when you take it continually every day, you can safely take this product on a daily basis, without any breaks.
See the product page to find out more about OptiBac 'One week flat'.