Marijuana Legalization and medical benefits in the society

Marijuana Legalization and medical benefits in the society. Congress has determined that marijuana is a dangerous drug and that the illegal distribution and sale of marijuana is a serious crime. Under the federal Controlled Substances Act (CSA) of 1970, marijuana is classified as a Schedule I substance. Colorado has legalized the use of marijuana for persons 21 and older at private homes. There is now a drive to legalize it everywhere, such as they have in Amsterdam. There is also a movement to make it legal for all residents, regardless of age. In fact, if you read the state sponsored ads, they advise people to keep marijuana out of the hands of those under 18, because their brain is still forming.
How does the state of Colorado (and several others) have the right to allow citizens to ingest and/or smoke this controlled substance in violation of a federal act? What happened to the Supremacy clause?