Lets look at scripture on this...
Follow the law of the land pot is illegal
Not where I live.
Rom. 13:1-5 Let every person be subject to the governing authorities. For there is no authority except from God, and those that exist have been instituted by God. Therefore whoever resists the authorities resists what God has appointed, and those who resist will incur judgment. For rulers are not a terror to good conduct, but to bad. Would you have no fear of the one who is in authority? Then do what is good, and you will receive his approval, for he is God's servant for your good. But if you do wrong, be afraid, for he does not bear the sword in vain. For he is the servant of God, an avenger who carries out God's wrath on the wrongdoer. Therefore one must be in subjection, not only to avoid God's wrath but also for the sake of conscience.
It is harmful to the body
So is fast food. Your point...?
I Cor. 3:16-17 Do you not know that you1 are God's temple and that God's Spirit dwells in you? If anyone destroys God's temple, God will destroy him. For God's temple is holy, and you are that temple.
We are also, as Christians, in a constant spiritual battle against Satan (Eph. 6:12). No person goes into battle unprepared, especially Christians. Marijuana's psychological effects include but are not limited to: paranoia, anxiety, fatigue, loss of ambition, lowered inhibitions, loss of concentration, and delusions and hallucinations; not exactly the traits of a godly warrior.
You could say the same for the "evils" of alcohol, yet most Christians drink.
When we realize that Satan is roaming around, seeking to "devour" us (I Pet. 5:8) and that we must be "self-controlled and alert", we must also realize that anything which weakens our resolve or chips away at the armor that God has given us is evil.
So basically everything is evil. The internet is evil too, then. We should all become monks in seclusion so we do not give in to the devil then. It is the only way.
I Thess. 5:22 tells us to abstain from the very APPEARANCE of evil. Phil. 4:8 tells us what to think on as Christians and since we usually end up DOING what we think about, this is an excellent scripture for this topic. Finally, Col. 3:5 tells us to put to death whatever belongs to our earthly nature. If we have died to self and live in Christ, we will not allow anything other than God to gain control of our lives. Marijuana WILL take over your life if you let it. I hope I've helped you and I hope you will continue to stand up for what is right and listen to scripture.