What factors effect the chemical stability and reduced potency over
time of pure pharmaceutical grade anhydrous caffeine powder?  Heat,
humidity?  Will it still have the normal effect on a human who ingests
it after 10 years in storage?  How can I measure the degradation?  Can
I tell simply by looking at the powder?  What is the worst case
reduction in strength in percent based on being unsealed and subject
to normal daily temperature fluctuations for 10 years in coastal
California?

You know what they say:
               "When in doubt,
                eat it and find out."