Sizemore, who had battled drug addiction since he was 15, was convicted in 2003 of assault and battery against his girlfriend, the former "Hollywood Madam" Heidi Fleiss. Sizemore was then sentenced to 17 months in jail and four months in drug treatment for repeatedly failing drug tests while on probation on March 25, 2005. Fleiss' restraining order against him had lapsed by the time they appeared together in the third season of Celebrity Rehab with Dr. Drew in 2010.
On May 8, 2007, while still on probation for a previous drug conviction, Sizemore was arrested outside the Four Points Sheraton hotel in Bakersfield, California for possession of methamphetamines. Police found what appeared to be two drug bags under 1 gram in weight of methamphetamine and three glass pipes used for inhaling methamphetamines that were coated with drug residue inside his 2004 Ford Mustang. On June 25, Sizemore was sentenced to 16 months in prison. The sentence was reduced to nine months because Sizemore already served 213 days in county jail unable to post bail. On March 27, 2009, Sizemore avoided jail-time in the above case for an alleged probation violation.
On May 28, 2009, Sizemore was arrested in Los Angeles on an outstanding warrant for drug charges. Sizemore and a friend were questioned by police and an outstanding $25,000 warrant for Sizemore was discovered. Sizemore did not have any narcotics in his possession, but was arrested on the outstanding 2007 warrant; he was later released.