Leary v. United States was a United States Supreme Court case that dealt with the issue of the constitutionality of the Marihuana Tax Act of 1937. The case was brought by Timothy Leary, who was convicted of violating the Act by failing to register with the Internal Revenue Service and pay the required tax on marijuana. Leary argued that the Act violated his Fifth Amendment right against self-incrimination, as it required him to provide information that could be used against him in a criminal prosecution. The Supreme Court held that the Marihuana Tax Act was constitutional, as it did not require Leary to incriminate himself. The Court reasoned that the Act was a valid exercise of Congress' power to tax, and that the registration and payment of the tax did not require Leary to provide any information that could be used against him in a criminal prosecution. The Court also held that the Act did not violate the Fifth Amendment, as it did not require Leary to provide any information that could be used against him in a criminal prosecution. In conclusion, the Supreme Court held that the Marihuana Tax Act was constitutional, and that Leary's Fifth Amendment rights were not violated by the Act. The Court's decision established that the Act was a valid exercise of Congress' power to tax, and that it did not require Leary to provide any information that could be used against him in a criminal prosecution.

Justice Field delivered the dissenting opinion in Leary v. United States, arguing that the Marihuana Tax Act of 1937 was unconstitutional as it violated the Fifth Amendment's protection against self-incrimination. He argued that by requiring individuals to register and pay a tax on marijuana, they were essentially incriminating themselves for violating federal law. Furthermore, he noted that since registration required disclosure of information regarding prior transactions with marihuana, this would be tantamount to forcing an individual to provide evidence against himself or herself in a criminal prosecution. Justice Field concluded his dissent by stating that Congress had exceeded its authority under the Constitution and thus should not have been allowed to pass such legislation which infringed upon an individual's right against self-incrimination guaranteed by the Fifth Amendment.