In the case of United States v. Doremus (249 U.S. 86 (1919)), the Federal government had been trying to regulate drug (cocaine, heroin, etc) use via the Harrison Act, which basically was a tax on drug sales.  The US Supreme Court upheld the Harrison Act on the basis that it did not exceed Congress' taxing powers. Congress' use of tax powers to regulate things that they could not regulate directly is similar to how they used the Interstate Commerce Clause to regulate behavior. Doremus was a druggist who wasn't following the Federal laws for filling out proper forms when dispensing drugs. The prima facie reason for the forms was to be able to tax drug sales, but the real reason was that they wanted to stop druggists from selling narcotics to junkies, and the forms provided evidence of those sales. So, in effect Congress was using the tax power to regulate narcotics sales, which is something that they did not have the power to regulate directly.