I smoked for 43 years and was up around 45 cigarettes a day. Tried everything I was aware of and/or my doctor recommended to quit which all resulted in expensive, spectacular failures.
Long story short, I had to go up to 24mg to completely quit smoking. It was definitely an overdose but that’s what it took to dump smoking completely. After a month or so I had to drop to the 16/18mg range. A while later it was down to 12mg. The nic just kept becoming too much and often felt like I was inhaling pepper spray, for lack of a better comparison, but by then I was into DIY.
There was a fantastic thread called, I think, the Baditude Appreciation Thread. Ran into somebody who was lowering their nic one mg at a time in there. Who woulda thunk it? Thought it was a great idea and started doing the same by mixing each new batch at one mg less than the one before. Vaped more for a day or two after each drop but then it all leveled out again.
I cheated going from 1mg to 0 by doing a batch of 1/2mg. Little did I know then, I can’t feel any nic below 3mg.

Stayed off the nic for several months and then got curious what a little nic would be like. Tried 1mg and it did zip. Same with 2mg. Mixed 3mg and hallelujah! I felt normal for the first time since I’d dropped to zero. I was calmer and less anxious, though hadn’t noticed it while I was at the zero mark. Husband said he had.
While I was off the nic, my BP had steadily increased up to 180. That was fun. After returning to the nic it dropped quite a bit, then steadily reduced back into the 120 range. Unfortunately I didn’t know nic could do that at the time so didn’t monitor the symptoms at the time, just put the pieces together after the fact. I now use 0 to 6mg, mostly 3mg and am happy there--physically and mentally.