This question would best be answered by your licensing board. The question of whether you can lose your job for refusing a drug test is governed by contract. I suspect that the policies of the employer dictate that you must submit to drug tests as a condition of your employment.
The consequences to your license would be governed by the licensing board. Many professions, including nursing I believe, are largely self-regulating. Meaning, your peers who gave you a license have a right to judge you and punish you if they believe you have acted unprofessionally or have committed some malfeasance.
As a general matter, if you have a prescription for the drugs which will be found in the drug test, it should be okay. Before you pee in the cup, they ask you to list any drugs that will be found in your system and, if necessary, to explain why. If other drugs (the alleged diverted narcotics) are found in your system, then that would be really bad. If it's possible that could happen - for any reason - you are right to not take the test. However, you can probably still lose your job and your license just for the suspicion and appearance of impropriety.
If your licensing board has an anonymous 800 number, I suggest you call it and ask for their advice.
I wish you the best of luck.
