New drug driving limits
Driving under the influence of drugs is an offence just like driving under the influence of alcohol: you're impaired, so you're not fit to be in charge of a potentially dangerous vehicle.
But until recently, there were no official driving limits for drugs (you could, however, be charged with driving under the influence of drugs if found to be impaired through a Field Impairment Test). From March 2015, new limits for a list of prescription and illegal drugs will come into force.
Drugs that will be under drug drive limits
Needless to say, just because there will now be an official drug driving limit for these drugs does not make driving under the influence of ANY amount of an illegal drug legal. The limits are actually set as low as they can be without picking up passive consumption, like standing near someone smoking cannabis.
These limits are being put into place to make it easier for police to rule out drug driving at the roadside, instead of having to confirm an uncertain Field Impairment Test with a blood test at a police station. They'll now do a saliva swab at the scene.
Just like with alcohol: if you're driving, just say no thanks.