According to the US Justice Department, in 2004, 27.9% of people serving time for drug offenses in state prisons had a possession charge as their most serious offense; 69.4% were serving time on trafficking offenses; and 2.7% were in for "other."
Source
Mumola, Christopher J., and Karberg, Jennifer C. Drug Use and Dependence, State and Federal Prisoners, 2004. Washington, DC: US Dept. of Justice Bureau of Justice Statistics, Oct. 2006. NCJ213530.