The
Federal Bureau of Prisons Quick Facts has a table of Federal Prison Population Over Time/Drug Offenders in BOP-Operated Facilities. In 2002, 54.7% of prisoners in federal BOP facilities were drug offenders. These statistics do not specify type of drug (pot, cocaine, etc.) or specific offense (possession, dealing, etc.). The high water mark was 61.3% in 1994.
By the way, the most alarming statistic (to me) on that page is the percentage of US federal prison population who are Mexican citizens: 16.3%, almost 1 out of 6. Aye carumba.