Schedule I is one of the classifications of drugs and other substances in the United States. It means that :
  • The drug has a high potential for abuse
  • The drug has no currently accepted medical use in treatment in the United States.
  • There is a lack of accepted safety for use of the drug or other substance under medical supervision.
  • Some Schedule I substances are heroin, LSD, marijuana, and methaqualone.

This information taken from the DEA website,

Schedule I is where all the really bad drugs go. A number of psychedelics have also made it on to the list. I believe that peyote and MDMA(Ecstasy) are also classified as Schedule I. By current criteria I can't figure out why alcohol and nicotine aren't on here, since they both have a high potential for abuse and don't have any accepted medical uses, and I can't figure out why marijuana is on here, since it does have accepted(well, sort of) medical use in the United States. Obviously this list is horribly biased, like most things in the American political system, and it would probably make sense to have something about addictive potential in the criteria. It would probably make more sense to just get rid of the War on Drugs altogether and try and educate people instead of throwing them in jail.

I've been informed by thecarp that alcohol does have a valid medical use, as an antidote for polyethylene glycol poisoning.