The most powerful argument against drug testing for welfare benefits stems from the constitutional standpoint. Organizations such as the ACLU have argued that the Fourth Amendment protects citizens from being searched without probable cause (ACLU).

The Fourth Amendment to the U.S. Constitution reads:

“The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.”

http://www.aclu.org