DOT Agency Regulations
Each DOT agency must comply with the DOT-wide regulations found in 49 CFR Part 40. However, each agency may institute their own rules regarding drug and alcohol testing. The primary differences between the agencies center around when to test, what additional drugs to test for, the rate of drug and alcohol testing required (including whether or not alcohol testing is required), and additional testing requirements for various specifications of employees such as certificate holders and safety-sensitive employees. Each agency maintains their regulations and a list of their safety-sensitive positions in individual publications of regulations under the CFR.
You can learn more about the regulatory requirements for each agency on their respective pages within this site.