Drug tests are only required for companies that do significant business with federal contracts. For everyone else it is optional, but courts have generally supported the right of employers to test employees for drug use. Most businesses use a third-party drug testing company for their drug tests, but there is no prohibition against doing it yourself with tests you buy yourself.
Since the tests are not legally required, how you do them is pretty much up to you. Many business owners feel it protects them from liability in cases of accidents, or that it is just good policy to ensure that employees are not under the influence of drugs.
The National Federation of Independent Businesses says the important thing is to be consistent, and whatever you do should be done with all employees. Random testing of current employees can be problematic if you do not have suspicion of drug use because you could open yourself to a discrimination suit if other employees are not also tested. Still, you can do it yourself instead of sending the tests off to a third party if you like.
Many companies do a drug test as a condition of employment for job applicants. If you are not dealing with federal contracts, you are under no obligation to do so, but as many as half of American companies give drug tests as conditions for employment. The NFIB recommends testing only those that you have offered a job to, and make that offer contingent on passing the drug test. It would not be practical to test every applicant, but once you have made a job offer, you can make the offer contingent on passing the drug test.
Of course an applicant can refuse a test, and you can then decide if you want to hire the employee or not. You have a right to not hire someone for either failing a drug test or for refusing to take one. You can also decide to hire someone who failed a drug test. The NFIB says the important thing is to treat everyone the same to avoid charges of discrimination.
It is also important to do this honestly. Don’t try to get a specimen from an employee without his or her knowledge. This is illegal and could get you involved in a lawsuit. As long as you are consistent in what you do, and treat all employees the same, you may administer your own drug tests to employees.