Marijuana contains a complex mixture of compounds including tetrahydrocannabinol (THC), the major psychoactive constituent, and cannabidiol (CBD), a nonpsychoactive constituent. We have shown previously that CBD pretreatment of mice increases brain levels of THC and have now further characterized this effect and determined whether the brain pharmacokinetics of other drugs are also affected. CBD pretreatment of mice (30-60 min) increased brain levels of THC nearly 3-fold, whereas CBD co-administration did not. Because marijuana is often consumed with other drugs, the influence of cannabinoids on the brain levels of several other drugs of abuse was also determined. CBD pretreatment of mice increased brain levels (2- to 4-fold) of subsequently administered cocaine as well as phencyclidine (PCP). Although CBD pretreatment increased blood and brain levels of cocaine comparably, blood levels of PCP were only modestly elevated (up to 50%). Behavioral tests indicated that the CBD-mediated increases in the brain levels of THC, cocaine, and PCP correlated with increased pharmacological responses. Pretreatment with THC instead of CBD could similarly increase brain levels of cocaine, PCP, and CBD, although with a lower potency than CBD. On the other hand, pretreatment of mice with CBD had no effect on the brain levels of several other drugs of abuse including morphine, methadone, or methylenedioxyphenyl-methamphetamine. These findings demonstrate that cannabinoids can increase the brain concentrations and pharmacological actions of several other drugs of abuse, thereby providing a biochemical basis for the common practice of using marijuana concurrently with such drugs.