According to the Controlled Substance Act:
§ 801. Congressional findings and declarations: controlled substances....
...(3) A major portion of the traffic in controlled substances flows through interstate and foreign commerce. Incidents of the traffic which are not an integral part of the interstate or foreign flow, such as manufacture, local distribution, and possession, nonetheless have a substantial and direct effect upon interstate commerce because -
(A) after manufacture, many controlled substances are transported in interstate commerce,
(B) controlled substances distributed locally usually have been transported in interstate commerce immediately before their distribution, and
(C) controlled substances possessed commonly flow through interstate commerce immediately prior to such possession.
(4) Local distribution and possession of controlled substances contribute to swelling the interstate traffic in such substances.
(5) Controlled substances manufactured and distributed intrastate cannot be differentiated from controlled substances manufactured and distributed interstate. Thus, it is not feasible to distinguish, in terms of controls, between controlled substances manufactured and distributed interstate and controlled substances manufactured and distributed intrastate.
(6) Federal control of the intrastate incidents of the traffic in controlled substances is essential to the effective control of the interstate incidents of such traffic.
(7) The United States is a party to the Single Convention on Narcotic Drugs, 1961, and other international conventions designed to establish effective control over international and domestic traffic in controlled substances.
From what I understand, this means that because of the Commerce Clause of the Constitution, which grants Congress the power:
To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes.
...Controlled Substances may not be exchanged through interstate commerce. Also, because it is impossible to determine whether drugs produced for intrastate commerce are also crossing state lines, then:
(5)...it is not feasible to distinguish, in terms of controls, between controlled substances manufactured and distributed interstate and controlled substances manufactured and distributed intrastate.
(6) Federal control of the intrastate incidents of the traffic in controlled substances is essential to the effective control of the interstate incidents of such traffic.
Therefore, no intrastate commerce is allowed. However, I don't understand how possession or manufacturing of drugs for personal consumption is covered in the Commerce Clause.
For example, let's say I wish to grow one cannabis plant for personal consumption. In what way does this affect the cannabis market in other states so that Congress has the power to prohibit it?