If you have been precribed a medication in the last four months and you are drug tested and come up positive for that in a drug test can they hold that against you?

Usually : Usually you are given a form before you have a drug test asking what medications you are on. You just need to write down all of your prescriptions. If you didn't get a form, just get proof that it was a prescribed medication.

Depends: If you are in a safety-sensitive job, like a bus driver or operating dangerous machinery, then taking a potentially impairing medication (like opioids or tranquillizers) without informing your employer may be a problem. In general, if you are not in that kind of job, most employers are not testing for prescription drugs anyway, and a prescription should be all you need if there are questions.