https://www.bop.gov/about/statistics/statistics_inmate_offen...
Only a little over 1/3rd are actually in prison for violent crime.
Here's a breakdown by most serious offense for my state's prison system:
https://www.doc.sc.gov/research/InmatePopulationStats/ASOF-M...
Which has 16% drugs, but also:
26% homicide, 12% Robbery, 11% Burglary, 9.5% Rape/Sexual assault, 6.6% Kidnapping, 6.4% Assault.
I wrote an blog post a while back about how prison populations skew so much towards the most violent crimes.