In the recent studies was most widely used illicit drug by drivers who drug & drive

The correct answer is Marijuana.

Alcohol is most widely used drug by drivers but it is not illegal therefore Marijuana is the correct answer here. Marijuana is a very common drug to take and has it's pro's and con's. When it comes to driving though it can slow down your reaction time and impair your ability to make decisions therefore it can be a dangerous drug to be on while driving.