49 CFR §382.211

No driver shall refuse to submit to a pre-employment controlled substance test required under § 382.301, a post-accident alcohol or controlled substance test required under § 382.303, a random alcohol or controlled substances test required under § 382.305, a reasonable suspicion alcohol or controlled substance test required under § 382.307, a return-to-duty alcohol or controlled substances test required under § 382.309, or a follow-up alcohol or controlled substance test required under § 382.311. No employer shall permit a driver who refuses to submit to such tests to perform or continue to perform safety-sensitive functions.