I'll have to dig up the regulations again, but it was implemented prior to Bush's ordered physical.
EDIT: Found some references:
In April 1972 the Pentagon implemented a drug-abuse testing programme that required officers on "extended active duty", including reservists such as Bush, to undergo at least one random drug test every year. The annual medical exam that year included a routine analysis of urine, a close examination of the nasal cavities and specific questions about drugs.... Sunday Times (UK), 6/17/00
and:
In 1972 he asked to be transferred to an Alabama unit so he could work on a Senate campaign for a friend of his father's. But some skeptics have speculated that Bush might have dropped out to avoid being tested for drugs. Which is where Air Force Regulation 160-23, also known as the Medical Service Drug Abuse Testing Program, comes in. The new drug-testing effort was officially launched by the Air Force on April 21, 1972, following a Jan. 11, 1972, directive issued by the Department of Defense. That initiative, in response to increased drug use among soldiers in Vietnam, instructed the military branches to "establish the requirement for a systematic drug abuse testing program of all military personnel on active duty, effective 1 July 1972."
It's true that in 1972 Bush was not on "active" duty: His Texas Guard unit was never mobilized. But according to Maj. Jeff Washburn, the chief of the National Guard's substance abuse program, a random drug-testing program was born out of that regulation and administered to guardsmen such as Bush. The random tests were unrelated to the scheduled annual physical exams, such as the one that Bush failed to take in 1972, a failure that resulted in his grounding.
http://sf.indymedia.org/news/2004/02/1676664.php____________________
Seems to me that knowing Bush has we do, even though there is a chance that his physical would not have included a drug test, we know Bush doesn't read, so if his friends told him about the new active duty regulation, I'd say it's a safe bet that he THOUGHT he would be tested and thus tried to avoid it.