In every state, it's illegal to drive while under the influence of drugs. However, state laws differ in terms of what qualifies as being "under the influence." Drug DUI laws are of two types: those based on actual impairment and those that make it illegal to drive with a certain concentration of a controlled substance in your system (called a "per se DUI"). Some states have both types of drug DUI laws, whereas other states just use impairment as the measure of intoxication for drug DUIs.
In all states, you can get a DUI for driving while actually impaired by drugs. But state laws differ with regard to how impaired a driver must be to be in violation of the DUI laws.
In most states, a DUI conviction based on impairment requires the prosecution to prove the driver was "substantially impaired" (the DUI standard in California) or impaired to the extent that it affected his or her ability to drive safely (the law in Nevada). In other words, the prosecution needs to show more than just minimal impairment.
However, in some states, you can be convicted of a type of impaired driving offense for operating a vehicle while even slightly impaired. For example, in Colorado, you can be convicted of a DWAI (driving while ability impaired) for slight impairment. And, in New York, you can be convicted of a DWAI if your ability to operate a vehicle as a "reasonable and prudent" driver has been impaired to "any extent."
With per se DUI laws, the prosecution doesn't have to worry about proving impairment. A per se conviction is based solely on the amount of drugs in the driver's system.
In a handful of states, the per se drug DUI laws actually specify the prohibited concentration of various controlled substances. For instance, in Ohio, a motorist can get a drug DUI for operating vehicle with a certain concentration—which is specified in nanograms—of amphetamine, cocaine, heroin, LSD, marijuana, methamphetamine, or PCP in his or her system. Similarly, Montana and Washington make it illegal to drive with five or more nanograms per milliliter of blood of THC (the main psychoactive ingredient in marijuana) in your system.
A long list of other states—including Georgia, Pennsylvania, Indiana, and Minnesota—take a stricter approach and just make it illegal to drive with any amount of a controlled substance in your system.
Generally, a person can be convicted of a drug DUI based on legal or prescription drug use. Per se drug DUI laws typically don't apply to legal drugs and medications (with the exception of marijuana in some states where it's legal). But impairment DUI laws normally apply to all intoxicating substances, including over-the-counter and prescription drugs.
However, some states—including Louisiana, Ohio, Iowa, and North Dakota—give drivers some leeway when it comes to medications that were prescribed by a doctor. In these states, a driver can establish a valid defense to a DUI charge by proving he or she was taking a prescription drug as directed by a physician.