And now this from another comment to the blog:
I don't think so. I believe the random drug-testing was performed during the flight physical, something new the early 70's. Remember the popularity of illegal drugs during this period; the military had a very limited interest in thinning its ranks, but didn't want to appear to approve drug use. If a pilot could clean up for his physical, he wasn't a problem.
As with the earlier post, this also sounds plausible, but I don't have any way of sorting things out. Besides, isn't this a job for the national media? They could easily find out exactly what tests were done in such a physical at the time. I'm sure that there are records of the physicals for other pilots that would show such a thing.
I should also add that I'm skeptical that Bush failed to take the physical because he was using drugs. From the sounds of it, even if there were a drug test, he could have just laid off the stuff long enough to pass the exam. That's is, unless he couldn't lay off the stuff that long, but I doubt if that is likely.