Christians must be in subjection to governmental “superior authorities,” obeying them as long as they do not demand that we violate God’s laws. (Romans 13:1; Acts 5:29) So since cannabis is illegal to consume, use, possess, cultivate, transfer or trade in most countries, it's use would not be considered being in a proper subjection to governmental “superior authorities”.
Even if it wasn't an illegal substance, another thing to consider concerning the smoking of marijuana is the effect that it could have on the smoker and those around him/her. The National Institute on Drug Abuse reports many of these effects on one's health: http://www.drugabuse.gov/publications/drugfacts/marijuana
With this in mind, consider these Bible principles:
2 Corinthians 7:1 counsels us to: "cleanse ourselves of every defilement of [the] flesh."
Even the second-hand smoke is harmful to others. The Bible says: "You must love your neighbor as yourself." (Matthew 22:39)
Cannot Be Equated to Alcohol
The use of marijuana cannot be likened to the drinking of products that contain alcohol since alcohol is currently legal to consume. Additionally, the Bible does not condemn the consumption of alcohol. The Bible’s view of alcoholic beverages is balanced. On the one hand, the Scriptures say that wine is a gift from God “that makes the heart of mortal man rejoice.” (Psalm 104:1, 15) Jesus himself often drank wine with his meals. (Matthew 11:18, 19) On the other hand, in condemning overindulgence the Bible uses the expressions “heavy drinking,” “excesses with wine, revelries, drinking matches,” ‘given to a lot of wine,’ and being “enslaved to a lot of wine.” (Luke 21:34; 1 Peter 4:3; 1 Timothy 3:8; Titus 2:3) The Bible forbids overindulgence in drinking. Drunkenness is a sin against God. (1 Corinthians 6:9-11)
As a result, drinking alcoholic beverages is not central in the lives of true Christians. They do not come dangerously close to drunkenness nor do they allow alcoholic beverages to impair or in any way interfere with their serving God with their whole soul and with a clear mind. (Matthew 22:37, 38)
The Bible helps to determine what constitutes a godly view of alcoholic beverages. Any amount of alcohol that may impair your “practical wisdom and thinking ability” as a Christian is too much alcohol. (Proverbs 3:21, 22) It also would seem unreasonable to conclude that a Christian would be “acceptable to God” if he drinks alcohol to the point of relinquishing his “power of reason”. (Romans 12:1)
For more, see:
MARIJUANA - Links to Information (INDEX; Watchtower Online Library)
Is Marijuana Really That Bad? (yp1 chap. 33 pp. 237-245; Watchtower Online Library)
What about marijuana—is it harmless? (rs p. 106-p. 112; Watchtower Online Library)
Does the Bible actually forbid the use of drugs for pleasure? (rs p. 106-p. 112; Watchtower Online Library)
Drugs - Links to Information (Defend Jehovah's Witnesses)
How Safe Is Marijuana? Why Say No to Drugs? Marijuana—A New Wonder Drug? (Jehovah's Witnesses Questions and Answers; Quotes from yp chap. 34 p. 278 and g01 7/8 p. 9)