The American Management Association conducted surveys of workplace surveillance and medical testing throughout the 1990s and into the 2000s. In 1991, drug testing of some kind was conducted by 63% of companies surveyed, growing to 81% in 1996, falling to 66% in 2000 and then to 62% in 2004. Drug testing of new hires was conducted by 48% of companies in 1991, growing to 68% in 1996, falling to 61% in 2000 and then to 54.5% in 2004. Drug testing of current employees was conducted by 52% of companies surveyed in 1991, rising to 70% in 1996, falling to 47% in 2000 and then to 44.3% in 2004.

Source

American Management Association, "AMA 2004 Workplace Testing Survey: Medical Testing" (New York, NY: American Management Association, 2004), p. 2.
http://www.amanet.org/trainin…
A 2000 AMA Survey: Workplace Testing: Medical Testing: Summary of Key Findings (New York, NY: American Management Association, 2000), p. 3.