Pennsylvania is one of a number of states that criminalizes a driver for DUI who has any amount of marijuana in their system. The "per se" law is set forth in 75 Pa.C.S.A. 3802 (d)(1). If you are stopped for any reason and subjected to a blood test which reveals even trace amounts of marijuana, you could be charged with DUI or "drugged" driving, subjecting you to a loss of your license and even jail time. You don't have to be impaired at the time you are stopped, or even driving poorly. For instance, you could be stopped at a red light and rear ended. If the police decide to test for drunk or drugged driving, even though you were without fault, if you have any amount of marijuana in your blood, you can be charged. You may have not even smoked the marijuana, but in a room where others have and you ingested the second hand smoke. Marijuana stays in your system for as long as thirty days. A person legally using marijuana for medicinal purposes is still subject to this law. So marijuana users, beware. I believe this law may untimately be challenged and overturned, but until then you are at risk of being arrested, tried and convicted.