According to a recent article in USA Today, as more and more Americans are using daily prescription drugs, whether prescribed by a doctor or not, more and more people are being arrested and charged with driving under the influence of drugs. A DUI case involving drugs, however, can be difficult to prove.
For one thing, it is difficult for law enforcement and prosecutors to prove impairment. Unlike blood-alcohol levels used to quantify and gauge intoxication while behind the wheel, there is no widespread similar system for prescription drugs. Nevada has developed a system that gives some numbers of certain drugs and levels of quantities in a person's system that may lead to impairment.
Even if a person is shown to have drugs in their system, however, it is difficult to prove they were impaired based on numbers because different people are affected in different ways by different types and amounts of medications and drugs. Certain prescription drugs can also stay in a person's system for up to a month.
In some cases, even if a person has a drug in their system that may warn on its label that someone who takes it may have trouble operating machinery, it doesn't mean that is the case for everyone. According to a recent article in the New York Times on employer drug testing, pain killers that are administered under the care and close supervision of a doctor can oftentimes help a person function without causing impairment.
Law enforcement often conducts field sobriety tests to determine impairment. Some states, such as Florida, also call drug recognition experts (DREs) to the scene to assess the level of impairment. DREs are police officers with special training in drug impairment.
DUIs involving prescription drugs difficult to prove (USA Today)