The answer, unfortunately, is that a prescription is not a license to drive under the influence of the prescription drug. Some prescription drugs have significant impairing effects - particularly drugs in the benzodiazepam family such as Clonopin (Clonazepam); and virtually any medication prescribed as a sleep aid, like Ambien; and most pain killers or muscle relaxants. All of these drugs have warning labels about the risk of driving or operating heavy machinery.  Of course, just using the medication and driving is not a crime. The government still has to prove beyond a reasonable doubt that the driver was impaired by the medication, just like in alcohol-DWI prosecutions.