if you buy a large quantity, keep the bottle closed tightly, and use a second bottle to keep only a couple of weeks supply at a time. Dont continously open the original bottle as moisture and just the air will cause the tablets to become more brittle.
Im not sure how much the humidity in the air effects the actual strength of the dose, but I see from my own bottle the tablets are much more brittle over a few months from constantly opening it.
Now I keep a few bottles with few tablets in it and refill them all at one time every 2 or 3 weeks.