What kind of work? Are you in a critical position? If you are, that gives you more power.
An employer doesn't want to deal with having to hire a new high-skilled employee.
But as has been said, record and document. Make them explicitly state, in writing or on tape, who they are and by what authority they are requiring you to take an experimental drug treatment as a condition of your employment.
Explain to them that you do not want to subject yourself to a drug testing program.
If/when they decide to terminate your employment, get it in writing. Don't quit. Then find a lawyer.