Reply #4: I wondered how it wouldn't be protected under the constitution

[Nikia]

At the time of alcohol prohibition, the government thought in order to have national prohibition that there needed to be a constitutional amendment. I don't see why consuming any substance would be any different.
As far as I understand, the national government strongly encouraged states to raise their drinking ages by denying them federal money if they did not.
For the early part of American history, states had much more power than they do now. They had more freedom in their laws regulating safety, health, and morals. After the Civil War, the government decided that there were some laws that shouldn't be left up to the states and that all citizens of the United States should be protected under the laws of the United States. As far as slavery and the rights of the freed slaves, this was a good thing. These post civil war laws were later applied to noble causes such as labor regulations and the production of safe products. The Constitution also has an interstate commerce clause, which was interpreted to include all commerce since almost some parts of any business come from out of state. I believe that the drug trade is also considered interstate commerce.
I would have thought that there might be some basis on the interpretation of right to privacy, but with the Bush court that might be struck down as well.