In McMahon v. United States et al., the Supreme Court ruled on whether a defendant's confession could be used as evidence if it was obtained while they were under the influence of drugs administered by law enforcement officials. The case involved two defendants, McMahon and O'Keefe, who had been arrested for mail theft and subsequently confessed to their crimes after being given "truth serum" injections by police officers. They later recanted these confessions in court, arguing that they were not made voluntarily but rather induced by the drug. The Supreme Court held that a confession is admissible only if it is freely and voluntarily given, without any form of coercion or intimidation from law enforcement officials. It further stated that administering drugs to induce a confession constitutes an inherently coercive practice which violates this principle. Therefore, the Court concluded that McMahon's and O'Keefe's confessions should have been excluded from their trial because they were not voluntary but coerced through drug-induced interrogation techniques.

In the dissenting opinion for McMahon v. United States, Justice Douglas argued that the majority's decision to uphold McMahon's conviction was a violation of his Fifth Amendment rights against self-incrimination. He contended that by allowing evidence obtained through an undercover agent who had gained McMahon's trust and then tricked him into confessing, the Court was essentially condoning entrapment and deception as legitimate law enforcement tactics. Furthermore, he believed this ruling set a dangerous precedent where any citizen could be coerced or manipulated into incriminating themselves without proper legal protections in place. This view held that such practices were fundamentally at odds with principles of fairness and justice enshrined in American jurisprudence.