Eighty percent of Americans get a daily jolt from caffeine, and now, as marijuana legalization has spread across the nation, some users and businesses have begun touting a combination of the two drugs. It's important to keep in mind that the caffeine in a regular cup of coffee or tea is a pretty powerful psychoactive drug, meaning it’s capable of affecting the mind, emotions and behavior, Dr. Sergi Ferre, a senior investigator at the National Institute on Drug Abuse (NIDA), told Live Science. Caffeine exerts its effects by blocking receptors in the brain for a neurotransmitter called adenosine, Ferre told Live Science.