One issue that arises in drug crimes, specifically those involving an allegation of the possession of marijuana is, are Medical Marijuana cards a defense to drug crime charges in the state of Pennsylvania. And the answer to this question is sometimes. Recently the Commonwealth of Pennsylvania has approved the medicinal use of marijuana. Intent to Deliver Unfortunately, some criminal defendants take that to mean that they are allowed to drive their vehicle on a roadway in Pennsylvania after ingesting pot so long as the marijuana was prescribed medically. Unfortunately, the law in the Commonwealth of Pennsylvania makes it illegal for any individual to operate a motor vehicle with any marijuana in his system. This is true regardless of whether or not the marijuana was prescribed by a doctor or purchased on a street corner. This is also true regardless of whether the marijuana has any actual effect on a criminal defendant's ability to safely drive his car. The rule regarding DUI and marijuana in Pennsylvania is that a defendant is guilty of DUI so long as marijuana is in the bloodstream of a driver, even if the marijuana is no longer impairing his driving.