What does it mean when people say marijuana is a Schedule I controlled substance?

In 1970, the Controlled Substances Act (CSA) was passed into federal law.  This act consolidated previous federal drug laws under one legislative umbrella and shifted responsibility for drug enforcement from the Treasury Department to the Department of Justice.  A new federal agency, the Drug Enforcement Administration (DEA), was created under the Department of Justice to enforce the CSA. 

The CSA also established five schedules for substances based on their demonstrated safety and abuse potential.  A drug is supposed to be placed in the highest schedule, Schedule I, if it meets the following criteria:

(A) The drug or other substance has high potential for abuse.
(B) The drug or other substance has no currently accepted medical use in treatment in the United States.

(C) There is a lack of accepted safety for use of the drug or other substance under medical supervision.

Schedule I drugs cannot be prescribed by doctors, and they cannot be produced or studied by researchers without DEA permission.

The Schedule I list includes heroin, marijuana, LSD, MDMA, psilocybin (mushrooms), peyote, and mescaline.  Many are surprised to learn that cocaine, opium, morphine, and oxycodone are Schedule II, and anabolic steroids are listed in Schedule III.  Alcohol and tobacco were exempted from the scheduling system altogether.