When my son was a minor (16) I found pot and paraphernalia and I did call the local police. They came right over, asked me to give them what I found, and they had a serious sit-down with-difficult child. They gave him a warning, and they confiscated the stuff, and said I could call them anytime if I found anything else, and that I could bring him right over to the station to get drug tested any time in the future. Of course, he was a minor, and they went above and beyond to scare him straight. It didn't work; difficult child had a holy meltdown after they left, he was blaming us for calling them, etc., etc. Too bad. Unfortunately, in a power struggle, usually the biggest idiot will keep doing self destructive things just to show he's more powerful, so he never stopped using while he was a minor in our home. In fact, he got worse.
Your child is not a minor, so you have the right to keep a drug free home.