But even if you pass a roadside check you could be hauled off to a police station for further checks for drugs including ecstasy, LSD, ketamine and heroin.
In January 2014 the new powers were announced and trials began in some areas of the UK, ready for the March 2, 2015 launch.
Victims of drivers who had used drugs and then killed or maimed, and their relatives, have welcomed the changes.
Road Safety Minister Robert Goodwill said: "This new law will save lives. "We know driving under the influence of drugs is extremely dangerous; it devastates families and ruins lives. "The government's message is clear - if you take drugs and drive, you are endangering yourself and others and you risk losing your licence and a conviction."
The advice if you take prescribed medication and drive is to make sure you know whether or not you will be affected by the law change. Better still don't drive.