Teen age daughter on drugs?

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I think that my 15 year old daughter is smoking pot. We have a great relationship, we talk about everything(I think). But lately she appears lazy and unattatched from the rest of the family. I have asked her about it and she says no. But I just have a feeling. What do I do from here. I have talked to her about drugs and how they affect your life not only now but in the long run also. What do I do?