Drugs! Hell yeah!
Keeping it simple is the key. It should be a feature that you can largely ignore to little I'll affect. But one of those cat problems that can get out of hand once started.
But first! I want to see trade expanded so I can send out caravans trying to addict towns in the area. I want doctors who are able to use these drugs to aid healing, or ease the last minutes of life.
I want teachers and schools.
I want to be able to set up a trade town that pilgrim traders will come to just to load their carts and empty their purses.
So keep it to three.
Mud Weed
Black Cap Mushrooms
Purple Twilight
Black caps grow in wooded areas and if your gatherer does not know what they are doing they maytake it hope by mistake. If taken by mistake they cause unhappy thoughts and is likely to attack anything near by.
If gathered knowingly they can be taken at parties or feasts and cause a happy thought. If taken before going into battle they get a buff of some kind. Like the Vikings.
Mud Weed is hippie elf stuff. You can buy it from them but it cost more each time you buy more. Your 7 dwarf pose won't have anything to do with it but some immigrants will arrive addicted. If you do not have a supply for them they will buy it from the elves when they arrive with their caravan. If one of their friends gets in a bad mood they will share the Mud Weed and they will feel better. But now they are hooked too. The down side is they will just sit there sometimes. When they should be working, or eating, or anything but standing there blankly.
You can grow your own, out side only, but the elves are not just going to give you seeds and the buds contain none.
Purple twilight only becomes a problem if you start dealing drugs yourself. Using Black Caps at parties can trigger it, growing your own Mud Weed or buying large amounts of it from the hippies.
It all starts when a human teaches a dwarf about this mountain flower. Highly addictive, causes many long work stoppages, eatting may stop, drinking may tripple, death if done to often or if too long between fixes. Needs a doctor to cure 50% return to use rate in the next season. Once a dwarf has made it a few seasons he won't likely use again, so you can lock him in a private garden where he farms the flower for trade to towns. Side affects include extra caravans arriving at odd times with few goods, war, dwarf immigation slow downs.
Ideas. There are many uses for these three items. They can be ignored, or used to advantage. They can get out of hand with carelessness.
I like it.