"The Supreme Court, on a number of occasions, has held that government-administered drug tests are searches under the Fourth Amendment.14 Therefore, the constitutionality of a law that requires an individual to pass a drug test before he may receive federal benefits likely will turn on whether the drug test is reasonable under the circumstances."
Source
Carpenter, David H., "Constitutional Analysis of Suspicionless Drug Testing Requirements for the Receipt of Governmental Benefits," Congressional Research Service (Washington, DC: Library of Congress, January 19, 2012), p. 2.
http://www.fas.org/sgp/crs/mi…
