In the case of Missouri v. McNeely (2012), the U.S. Supreme Court ruled that law enforcement must generally obtain a warrant before subjecting a drunken-driving suspect to a blood test, and that natural dissipation of alcohol in the bloodstream does not constitute an exigency justifying departure from this requirement. The court held that while circumstances may make obtaining a warrant impractical such as when delay would significantly undermine the efficacy of search or seizure, these exceptions do not apply categorically to every case involving drunk driving. Instead, they require careful consideration and review based on totality of circumstances unique to each situation. This decision upheld Fourth Amendment protections against unreasonable searches and seizures.

In the dissenting opinion for Missouri v. McNeely, four justices argued that the natural dissipation of alcohol in the bloodstream presents a per se exigency that justifies an exception to the Fourth Amendment's warrant requirement for nonconsensual blood testing in all drunk-driving cases. They contended that this approach would provide much-needed guidance to law enforcement officers and protect individuals from arbitrary invasions of privacy. The dissenters believed it was unnecessary and unwise to conduct a case-by-case analysis on whether an officer could reasonably obtain a warrant within a timeframe permitting effective collection and analysis of evidence, as majority suggested. Instead, they proposed adopting clear rules which can be easily applied by police officers under pressure-filled circumstances.