While adding certain features I stumbled upon Mudders Milk and felt that it would be really interesting to add Drugs to minecraft. Now while you can't add block ids a couple things happen to look like certain drugs. e.g. Sugar - Cocaine, Reeds - Weed, Brown/Red Mushrooms - Shrooms, etc. There isn't many visual effects you could do for them but like Mudder's milk you could add things such as text editing, staggering, health damage, addiction, overuse. For addiction the plugin could detect how many of a certain drug have been used in a day (MC Day) and if a limit is reached declare addiction and begin damaging the player if it isn't taken every hour or so. To cure addiction there could be a command or an item that could be used. To reduce CPU intensity, the state of addiction shouldn't be stored and should just be dismissed upon log out. To be realistic certain drugs shouldn't be addictive mainly weed.