Dr. Declan Bedford:

I do not have all of the data for that but I know that a unit of alcohol takes approximately an hour to get out. It is slower if a person has been eating and so on, and it depends on other factors. Cannabis can stay there for several hours. The trouble is when people are smoking, they do not know how much they have taken. If someone takes a pint of Guinness, they have taken two units, so they know that exactly, but when they take a drag of a cigarette or a joint, they do not know what they have taken.