I'm 14 years old and female. My best friend (who's a guy) has just started smoking cigs and will start smoking weed soon. He doesn't know how much I care about him. Since I am an only child, I view him as my older brother (he's older than me by half a year). I want to make him stop, but I don't want to ruin a friendship. It scares me what he's doing to himself, and it makes me want to cry sometimes. So my question is: How can I make him see how much I care about him and make him stop?
Let's start with a very basic fact. You can't make anyone be good. You can encourage a person to do the right thing. You can rebuke him when he does the wrong thing. But you can't make him make good decisions. God doesn't do it to us, even though He wants all of us to be saved (II Peter 3:9), and we certainly can't do it to other people.
The second difficult is that he already has demonstrated that he doesn't care about the law. Smoking cigarettes is illegal before the age of 21, and he is choosing to break the law at 14. Therefore, reasoning with him about the legality of what he is doing wont make an impact on him.
There are some resent scientific studies that he should know about. See:
Perhaps health reasons might sway him, though since he has already chosen to smoke cigarettes and that is very well documented to cause lung cancer, even these facts will likely not change his mind.
About all you have left is to tell him that you would hate to lose his friendship because he is choosing to harm himself. Tell him that you don't want to watch your best friend go downhill. Likely it will make him pause, but the odds are that it won't stop him.
It really comes down to that there has to be a greater reason to him to not smoke than to smoke. I can't guess what would get through to him. There is no one right answer -- if there was you and I could solve the world's drug problems.
But I want you to understand a very important concept. If he goes, it will be his choice, not yours. It will be he who ruins the friendship, not you. It is just as our sins ruin our relationship with God and it isn't God's fault. "Behold, the LORD'S hand is not shortened, That it cannot save; Nor His ear heavy, That it cannot hear. But your iniquities have separated you from your God; And your sins have hidden His face from you, So that He will not hear" (Isaiah 59:1-2).
I hope he listens to his best friend, but if he doesn't don't chase after him. That will only lead to you being dragged down to his level. "Do not be deceived: "Evil company corrupts good habits"" (I Corinthians 15:33).