An employer can drug test employees even if it did not warn them previously (e.g. when they first applied or interviewed for the position) that it would do so. It may do so so long as it is not discriminating in who it tests (e.g. testing minority employees, not white employees; testing men, not women), which means it can drug test everyone; drug test on a random basis; test people in certain positions (e.g. drivers); or test people if certain events or behavior occur (e.g. if an employee has an accident at work). In your state, there is no mandatory notice period set by law.