Watchtower dogma on tobacco and narcotics use is dogma. Tobacco use is prohibited as it is "unclean" and the body is a "temple to worship Jehovah". Illegal narcotics are shunned because the Greek word for "spiritism" translated into "druggery". Supposedly use of narcotics "opens your mind up to the demons".
I know chewing on betel but has been denounced by the Watchtower. I would imagine drinking tea from the coca plant would be viewed similarly even though it is nothing like snorting several lines of cocaine. I would be surprised if marijuana use that is prescribed by a doctor in a state that it is legal is accepted by the Watchtower. I would imagine that it's all purely an image thing when it comes to using the Devil's lettuce.