I love this! I have some custom healing rules in my homebrew which necessitate a potent magical herb for healing, and I'd told them already it had addictive properties, so I'll definitely use this idea. I think you could also use some withdrawal rules, especially if someone's incredibly addicted and quits cold turkey. Maybe disadvantage as per the Poisoned condition? Could nerf it a little, depending on the severity of the addiction, but I think it'd make for interesting roleplay opportunities.

Here's mine:

Sunroot - an amber liquid once distilled from the exceptionally valuable sunroot herb. Potent painkiller and relaxant. Difficult to find, given sunroot's medical value.

Effects - Advantage on constitution and wisdom saving throws, and 10 temporary HP. Lasts two hours.

Addiction - Upon each dose roll for an addiction saving throw, Wisdom vs DC 8+number of doses previously not resisted (max 10) to resist. Upon each long rest, roll for an addiction saving throw, identical to above; success removes 1 DC from the next addiction save, failure results in disadvantage on constitution and wisdom saving throws.

DC 8-10 - Works as normal, but permanent disadvantage on constitution and wisdom checks/saving throws.

DC 11-15 - Effects only last an hour, but permanent disadvantage on constitution and wisdom checks/saving throws, and -3 to perception and sense motive, which lasts two hours after the drug wears off.

DC 16-18 - Effects only last half an hour, but permanent disadvantage on constitution and wisdom checks/saving throws, and permanent -4 to constitution and wisdom.

I'm not so sure about the second/third stage addiction. I think -2 is too little and -4 is too much, but I'm not sure what to do instead. The drug is supposed to be crippling if someone develops an addiction to it, it's definitely a little less forgiving.