What do you do when your child is doing drugs?

There are some will say a child has to experience things in life to know for self.You have some that will stand idle and do nothing and they lose their child to the streets,prison,even death.Do you call the police?Do you punish with grounding or physical punishment[spanking]?Do you just talk with them?What is the correct response.As I continue to stress the ultimate goal is to find solutions to save the children.