Unfortunately the current US federal controlled substances laws are a kafkaesque nightmare. There is something which is often referred to as "the drug exception to the Constitution" or "the drug exception to common sense". In this case, there is a "drug exception" to the normal requirements for a conspiracy conviction.

Under US federal law, most conspiracy crimes require both an agreement between two or more people and some sort of action intended to work towards completing the plan by any of the people involved with the plan. For instance the law banning "Conspiracy to commit an offense or defraud the United States" (18 U.S.C. § 371) states that it is a crime:

If two or more persons conspire either to commit any offense against the United States, or to defraud the United States, or any agency thereof in any manner or for any purpose, and one or more of such persons do any act to effect the object of the conspiracy...

And this is what is commonly thought of as the requirements for a conspiracy conviction.

However, in the case of crimes involving controlled substances, the law is different and removes the "any act to effect the object" language: (21 U.S.C. § 846)

Any person who attempts or conspires to commit any offense defined in this subchapter shall be subject to the same penalties as those prescribed for the offense, the commission of which was the object of the attempt or conspiracy.

This makes simply talking about committing a drug crime (for instance talking about buying cannabis or talking about growing cannabis) potentially criminal. Although it is important to note that a prosecutor has to prove that an actual conspiracy or "agreement" had taken place, if two people agree to possess/distribute/manufacture a controlled substance, then they are guilty under this statute. The implication that the individual is as guilty and punishable for talking about buying cannabis as they are for actually buying it can certainly seem chilling.

The immediate reaction of most who hear this is disbelief. A typical response might be "That can't be constitutional, I have a right to talk about whatever I want to talk about so long as I don't actually do anything else."

But, unfortunately, there have been several challenges to this law and the Supreme Court of the United states ruled unanimously in 1994 in the case United States v. Shabani, 513 U.S. 10 (1994) that the federal law was enforceable as it was written. A lower court, the more civil-rights-oriented US Ninth Circuit of the West Coast, had ruled that there had to be some actual activity beyond talking before a felony had been committed.

Justice O'Connor, writing for the unanimous Supreme Court stated it very clearly:

What the Ninth Circuit failed to recognize we now make explicit: In order to establish a violation of 21 U.S.C. § 846, the Government need not prove the commission of any overt acts in furtherance of the conspiracy. ( US vs Shabani, 1994 http://supct.law.cornell.edu/supct/html/93-981.ZS.html )