My Employer Of 15 Years Recently Permantly Laid Off Close

My employer of 15 years recently permantly laid off close to 500 of us..so I'm looking for a new job..my question concerns a medical issue..I have a prescription/medication in my name that I take (generic Percocet)..my question is: Can a new employer hold it against me if I should continue taking it and it comes up on drug screen? Or should I just stop taking it so no flags be raised or questions asked?..If so, How long does it stay in your system after you stop taking it(?)...even if I've been taking it for a year now?