I've used protein powder and creatine, granted both can technically absorb water, but I've not seen this have any effect over months of time. The protein/creatine stays sealed till I use it, then the caps are sealed from moisture when you close them. As long as you use something with the same consistency of the powder you're mixing them with, to get even distribution, you should come out with caps fairly close to what you intended... my 25mg caps are usually +-2, maybe 3mg max per cap. Usually plus, since I intentionally overdose my caps to make sure I don't loose any or have several underdosed caps in a day!