Speak to your friend first. Find out why they feel the need to take drugs. If they want help tell them to speak to their GP as the first point of contact. I don't think reporting them is the best thing to do rite now. They dont need to lose their job and be on drugs as well, could make things worse. Could advise them that it may be noticeable and they could end up losing their job, mite make a difference. At the end of the day, I think the best thing you can do is be a loyal friend who is there to help if required. If your friend doesnt want help there is nothing you can do to prevent them from beginning a downward spiral.
Lady Melina