In the 1968 case Leary v. United States, the U.S Supreme Court ruled in favor of Timothy Leary, overturning his conviction for violation of the Marihuana Tax Act of 1937. The court held that this act was unconstitutional as it violated Leary's Fifth Amendment right against self-incrimination. This decision came about because under the Marihuana Tax Act, one had to report their possession and pay a tax on marijuana transactions; however, complying with these requirements would have essentially forced individuals to admit to possessing an illegal substance (since marijuana was illegal in many states), thereby incriminating themselves. Therefore, by requiring such admissions from those involved in drug transactions without providing immunity from prosecution at both state and federal levels, the law infringed upon constitutional rights against self-incrimination.

In the dissenting opinion for Leary v. United States, Justice Black argued that Congress had a rational basis to classify marijuana as it did under the Marihuana Tax Act of 1937 and thus its decision should be respected by the Court. He contended that there was sufficient evidence showing potential harm from marijuana use, including addiction and psychotic reactions in some users. Furthermore, he disagreed with the majority's interpretation of self-incrimination rights under Fifth Amendment, asserting that not all questions leading to incriminating answers are prohibited; only those which carry substantial hazards of self-incrimination at time they are asked fall within constitutional protection against compelled testimonial disclosure. Therefore, according to him, Timothy Leary’s conviction should have been upheld because his claim about being forced into self-incrimination when complying with tax act was invalid.