What happens if you get reported to work for potential drug use?

Yes, you can be fired for failing a drug test,,, but,,, the exact details of such would be a matter of company policy, some companies will allow you the choice of some sort of rehab before firing you,, some will fire you immediately no questions asked,,

If it is a condition of employment that you submit to a drug test then... yes.

If your employer prohibits illegal drugs then you can be fired 

Yes.                                      .

Yes your employer could request you take a drug test, refuse or fail you are very likely to be sacked