...It is readily apparent that the Government has a compelling interest in ensuring that front-line interdiction personnel are physically fit, and have unimpeachable integrity and judgment. Indeed, the Government's interest here is at least as important as its interest in searching travelers entering the country. We have long held that travelers seeking to enter the country may be stopped and required to submit to a routine search without probable cause, or even founded suspicion, "because of national self protection reasonably requiring one entering the country to identify himself as entitled to come in, and his belongings as effects which may be lawfully brought in." Carroll v. United States, 267 U.S. 132, 154, 45 S.Ct. 280, 285, 69 L.Ed. 543 (1925). See also United States v. Montoya de Hernandez, supra, 473 U.S., at 538, 105 S.Ct., at 3308; United States v. Ramsey, supra, 431 U.S., at 617-619, 97 S.Ct., at 1979-1980. This national interest in self-protection could be irreparably damaged if those charged with safeguarding it were, because of their own drug use, unsympathetic to their mission of interdicting narcotics. A drug user's indifference to the Service's basic mission or, even worse, his active complicity with the malefactors, can facilitate importation of sizable drug shipments or block apprehension of dangerous criminals. The public interest demands effective measures to bar drug users from positions directly involving the interdiction of illegal drugs.
The public interest likewise demands effective measures to prevent the promotion of drug users to positions that require the incumbent to carry a firearm, even if the incumbent is not engaged directly in the interdiction of drugs. Customs employees who may use deadly force plainly "discharge duties fraught with such risks of injury to others that even a momentary lapse of attention can have disastrous consequences." Ante, at 1419. We agree with the Government that the public should not bear the risk that employees who may suffer from impaired perception and judgment will be promoted to positions where they may need to employ deadly force. Indeed, ensuring against the creation of this dangerous risk will itself further Fourth Amendment values, as the use of deadly force may violate the Fourth Amendment in certain circumstances. See Tennessee v. Garner, 471 U.S. 1, 7-12, 105 S.Ct. 1694, 1699-1701, 85 L.Ed.2d 1 (1985).
Against these valid public interests we must weigh the interference with individual liberty that results from requiring these classes of employees to undergo a urine test. The interference with individual privacy that results from the collection of a urine sample for subsequent chemical analysis could be substantial in some circumstances. Ante, at 1418. We have recognized, however, that the "operational realities of the workplace" may render entirely reasonable certain work-related intrusions by supervisors and co-workers that might be viewed as unreasonable in other contexts. See O'Connor v. Ortega, 480 U.S., at 717, 107 S.Ct., at 1497; id., at 732, 107 S.Ct., at 1505 (SCALIA, J., concurring in judgment). While these operational realities will rarely affect an employee's expectations of privacy with respect to searches of his person, or of personal effects that the employee may bring to the workplace, id., at 716, 725, 107 S.Ct., at 1497, 1501, it is plain that certain forms of public employment may diminish privacy expectations even with respect to such personal searches....
