I think if they suspected you were DUI, they would have to perform a urine test or blood test, which of course you would refuse (always, unless you knew your system was clean) but I think when you are on medication like pain pills, you have what they call "pinpoint" pupils, when your pupils are real small rather than dilated, but of course if it where daytime when you got pulled over, your pupils would be small anyways because of the sunlight.