In reply to Helen R:
Never ever known anyone to have long-term side effects, not even short-term side-effects (usually headaches from caffeine withdrawl).
At the same time there is a roughly 20-25% chance you are on placebo anyway. Also you get one of the most thorough medical checkups you will ever get, including urinalysis, blood analysis, HIV, DOA, ECG, EEG, (even MRI and PET on one occaison), constant blood and heart monitoring for the duration, etc etc etc. To top it all off you are cleansed of all caffeine containing, alcohol and illegal drug products for the duration.
Can't say I come out of these things feeling refreshed as they can take up to a litre of blood (typical blood donation), but I do get peace-of-mind from the fact that if anything was slightly unusual about my body chemistry then it would have shown up in the trial.
Lastly, the trials are usually testing minimal amounts of the drug being administers - it's not like they're dosing you up to the eyeballs.
But, each to their own.