Yeah drugs definitely degrade. Natural stuff like buds, mushrooms etc really only have a 12 month lifespan unless you take some real extra measures. I stored mushrooms 'properly' in rice and in the dark but they were quite weak after 6 months or so. There are better ways than that. I've heard some people claim they stored mushrooms for years with only minimal degradation.
Synthetic stuff seems to last a few years assuming it isn't exposed to heat, moisture, oxygen, and UV.
I'd plan to use any drugs I buy within 12 months just personally.