In 2005, Quest Diagnostics reported that 4.1% of all drug tests were positive. When the results are reported by specific reasons for testing we learn the following about positivity rates for the general workforce:

Periodic 2.4%
Pre-employment 3.9%
Return-to-duty 6.0%
Post-accident 5.8%
Random 6.6%
Follow-up 9.6%
For cause/reasonable suspicion 28.3%

Therefore, when drug use is suspected, as in the case of for cause or reasonable suspicion tests, drug use is often present.1

Drug Users Avoid Drug Testing - data from the federal government indicates that a high percentage of drug users will avoid working for companies that conduct drug testing. According to a survey of full-time employed drug users, 40% said they were less likely to work for a company that conducted random drug testing and 30% said they were less likely to work for a company that conducted pre-employment drug testing.2

As a Deterrent - in a survey of Navy personnel, 83% indicated that drug testing was the number one deterrent to drug use, and 27% said they would resume using drugs if the Navy discontinued its drug testing program.3

In Reducing Accidents - a number of major workplace studies measuring the accident rates of companies before and after implementing drug-testing bear this out. One of the most prominent of those studies involved the Southern Pacific Railroad. Following the implementation of drug testing, the company experienced an incredible drop in accidents that resulted in injuries, from 2,234 the year before drug testing was introduced to 322 in the first six months of the fourth year of the program. Annualized, this is a 71% decrease in accidents. The Southern Pacific Railroad study is indicative of the impact that drug testing can and does have on accident rates among companies of all sizes, representing virtually every industry.4

What Employers Think - a survey of business executives underscored the benefits of drug testing from a company's perspective:5

77% said drug testing resulted in a better caliber of job applicants.
58% noticed a "better public image" since implementing drug testing.
56% said they were experiencing fewer workplace drug problems.
54% had noticed an improvement in employee morale.