A colleague has taken drugs in my presence and sometimes in my house. Am I guilty of anything and/or do I need to report them to anyone?

One of the questions you get asked a lot as a business-crime lawyer is how should you react if someone takes drugs in front of you, particularly if they do it in your house. Luckily I have never had to deal with this moral conundrum but it has happened to a couple of my colleagues when clients have inexplicably decided that it would be good idea to share some of their personal stash of cocaine with the partner in charge of their case.