PROHIBITED DRUGS: Department employees, either on or off duty, will not: use any prohibited drug unless properly prescribed by a physician and not ______________ the amount prescribed.

beyond

The question is about the use of prohibited drugs by department employees. The statement is saying that employees are not allowed to use any prohibited drug unless it is properly prescribed by a physician. The blank in the sentence is referring to the amount of the drug that can be used. The correct word to fill in the blank is "beyond", meaning that the employee should not use more than the amount prescribed by the physician.