Well, for a start, you need to sit down CALMLY with her and discuss this --- see what she thinks and feels about this, and whether this is experimentation out of curiosity or if it relates to problems and troubles she may be trying to evade. See what she knows of the risks of using it ( there's recently been more emphasis, for instance, on the risks of a psychotic illness which are increased by using dagga ). Encourage her to see a good local shrink, perhaps best a psychologist in this case, to assess her and the situation, and advise you both on how best to handle this further. If she's 18 she may be considered to have the legal right to be foolish, but not with an illegal drug !