The stuff has only been out a few months, so you won't find any that's sat long enough to lose any effectiveness. I am taking kilo's M1t now from a bottle that I opened about 5 months ago. Gains are equal or better. The stuff should be good for 2 years, then gradually lose a little potency. I dont think opening it matters, its not vaccum sealed. You can try freezing it, that may prolong it.