Yes it is. The Bible may not say “Though shalt not do drugs,” however, the Bible makes it clear that we are to live holy and honorable lives and doing drugs is neither holy nor honorable. Drugs are not only illegal but they destroy the mind, the body, and the relationships with everyone around the drug user, and all of these totally conflict with God’s will for mankind. So Christians are obligated to refrain from all drug use, and I personally believe that includes smoking weed.