http://www.realchange.org/bushjr.htm#vietnam8. Skipped all his medical exams after they started drug tests.
In April 1972, the military started including routine drug tests in servicemen's annual physical exam, including urinalysis, questions about drugs and "a close examination of the nasal cavities" (for cocaine). According to the regulation, the medical took place in the month after the serviceman's birthday. For George W. Bush, this meant August 1972.
It was May, 1972 -- one month after the drug testing was announced -- that Bush stopped attending Guard duty. In August 1972, he was suspended from flight duty for failing to take his physical. (Click here to see the document.) A Bush campaign spokesman confirmed to the London Sunday Times that Bush knew he would be suspended. "He knew the suspension would have to take place." Bush never flew again, even though he returned to his Houston base where Guard pilots flew thousands of hours in the F-102 during 1973. The only barrier to him flying again was a medical exam (and his lack of attendance).
Careful readers will recall that when Bush issued his partial denial of drug use, he said (or implied) that he hadn't used them since 1974, but he pointedly refused to deny drug use before then, i.e. during his military service.
http://www.realchange.org/bushjr.htm#vietnam