49 CFR §655.49

1. (a)Each employer shall require a covered employee to submit to a post-accident drug and alcohol test required under § 655.44, a random drug and alcohol test required under § 655.45, a reasonable suspicion drug and alcohol test required under § 655.43, or a follow-up drug and alcohol test required under § 655.47. No employer shall permit an employee who refuses to submit to such a test to perform or continue to perform safety-sensitive functions.
2. (b)When an employee refuses to submit to a drug or alcohol test, the employer shall follow the procedures outlined in 49 CFR Part 40.