It's possible to be charged for driving under the influence of drugs even if those drugs are given to you by prescription. For many people, there is a disconnect between the medications taken by prescription and illicit drug use, but the truth is that all medications and drugs of all types have the potential to cause side effects that can affect your ability to drive safely.
On the side of most prescription medication bottles or packets there is information that describes the potential side effects. Drugs known to make people tired will state not to drive because of the potential for drowsiness. One thing that may be harder to predict is how the drugs will affect you over the course of time, though. The body can get used to the drug in its system, but once in a while, old side effects could show themselves again. They have the potential to cause serious injuries, especially if you're driving at the time.
For police, this is a difficult situation. On one hand, you can't take an innocent person to jail for something he or she was unaware would happen. On the other, drugs of any kind can make a person act in an intoxicated manner.
Remember, prescription drugs have many side effects that can affect how you drive. They may make you drowsy, hallucinate or aggressive. You may feel exhausted or energized. Some may cause you to feel overheated, which is a distraction when you're driving, while others have other side effects that can affect your ability to drive safely.
You can defend yourself if you're accused of a DUI when you're taking prescription medications. Not all side effects are common, and unusual effects may not have been discussed with you.
Source: The New York Times, "Drivers on Prescription Drugs Are Hard to Convict," Abby Goodnough, Katie Zezima, accessed Aug. 22, 2017