People always say marijuana is "natural" and God wouldn't have put it on the Earth if we weren't supposed to have it.
I'll say a few things with this: Every "drug" is originally natural. So are poisonous plants. We live on a fallen Earth, and not just man was corrupted by sin.
Since we have these promises, beloved, let us cleanse ourselves from every defilement of body and spirit, and make holiness perfect in the fear of God.
2 Corinthians 7:1
Be sober, be vigilant; because your adversary the devil, as a roaring lion, walketh about, seeking whom he may devour. Peters 5:8
[FONT=Arial,helvitica]There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it. Corinthians 10:13
[/FONT][FONT=Arial,helvitica]Thou shalt have no other gods before me. Exodus 20:3 (addiction is placed before God, it becomes your god/idol)
[/FONT][FONT=Arial,helvitica]Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen.[/FONT] Romans 1:25
Overall, it is your choice, but to continue to do it if you know it is wrong is a sin.
[FONT=Arial,helvitica]Jesus said unto him, It is written again, Thou shalt not tempt the Lord thy God. Mathew 4:7[/FONT]
Also, if you need background, I used to be a huge pothead in high school. Actually, I've experimented with a lot of different drugs. Either way, it does nothing but harm, but those that smoke it can't see that til they quit. It's up to you, but I wouldn't recommend trying to prove it is biblically acceptable on any level.