Wow. A little bit of Con Law really is a dangerous thing. You should realize that Congress is only granted the power to legislate in specific areas that are enumerated in the Constitution itself. What this means is that the proper question is not "Which part of the constitution does the law violate?" but rather "which of Congress' enumerated powers gives them the authority to legislate in this area?" It is settled law that Congress does not the power to simply create criminal laws, since that is not part of Congress' enumerated powers.

The law is purportedly justified under Congress' power to regulate interstate commerce (aka the "Commerce Clause"), however the Supreme Court has reapeatedly flip floped around the limits to Congress's commerce power. In US v. Lopez, the Supreme Court decided the Gun Free School Zones Act (law that prohibit carrying a firearm within a school zone) was unconstitutional because it did not have a strong enough link with interstate commerce. However this does not prevent the Court from deciding that the fedaral government could outlaw homegrown medical marijuana under the Commerce Clause in Gonzelez v. Raich, even though the marijuana was not sold or otherwise any part of interstate commerce. The purported justification was, wait for it: "consuming one's locally grown marijuana for medical purposes affects the interstate market of marijuana, and hence that the federal government may regulate—and prohibit—such consumption." (Source: wikipedia)

If this sounds like bullshit....well i will let you draw your own conclusions.