You can be but there are systems in place to detect if you are impaired while driving. It may seem subjective but you should understand that any such illegal drug in your system is cause for concern because a) it should not be there and b) you should not be operating a moving vehicle while it is in your system.  Think of it as driving while buzzed, which is completely illegal. If the police have cause to pull you over for erratic driving, it won't be good for you if tests show it is in your system.