How can a prosecutor prove impairment in driving under the influence of prescription medication?

This would be rather difficult to prove. i would say that the best way is to question the person about how many pills he took, and ask their doctor/pharmicist what the dosage is. When he was pulled over, they should have had a tox. screen (toxicology screen) done to see how many drugs they have taken. if he wasnt pulled over, find a material wittness. go from there. hope this helps!