Can an employer give an employee a random drug test because they receive an anonymous call saying the employee was using drugs?

If the employer already has a drug testing policy in place, the anonymous call would not matter. It would be unusual, and potentially actionable, for an employer to suddenly start drug testing because of a tip.

I lecture on drug testing statutes for the Florida Bar. Under the Florida Drug Free Workplace Act, an employer may drug test an employee without warning based on a "reasonable suspicion" that an employee is using drugs or is intoxicated in the workplace. Reasonable suspicion may be based on a variety of factors, including a random phone call, assuming there is some other evidence to support the conclusion that the employee is using an illegal substance, such as observations of the employee in the workplace, slurred speech, red eyes, etc. If the employee requests, the employer must put the basis for the reasonable suspicion in writing. So although a random phone call alone would probably not support a reasonable suspicion of drug use, it could be one of the factors considered by the employer.