It's not that high for state and federal PRISON, but it may be that high for local county jails. Drug offenders made up %8 of the prison population in 1980, but now account for over %26.
Here's another figure for you: as of 1995, more people had been sentenced under California's "Three Stikes" law for simple marijuana possession than for murder, rape, and kidnapping combined, and more for drug possession generally than for ALL violent offenses.