It's the same as with alcohol. What we have done is create a parallel process. When an officer stops a driver, his first thing to look for may be alcohol. All he needs is a suspicion that the driver has consumed alcohol. That leads to the next step.
In this particular case, if there's a suspicion that drugs have been used, then it leads to the next step. The suspicion leads to the next step, but it's the next step that leads to the charge.