And sometimes even pack a day smokers have a hard time with higher levels, if they went outdoors to smoke, but vape indoors, which means they can vape constantly. I smoked a pack a day for about 3-5 yrs before quitting, and before that, a lot more, but when I first tried 18mg when I first started vaping, it made me sick as a dog. 12mg did too; I had to go all the way down to 6mg to get rid of the nausea, and then go back up to 8 or 9 mg once I'd actually quit smoking. Still can't do more than 10mg for more than an hour, or get nauseated again. Usually I stick with 9mg for daytime, 6-7 mg at night.
Shaking hands is most definitely a sign of too much. Stomach pains/nausea would be, too. Basically if it makes you dizzy to vape it, it's more than you're used to, and has a strong potential of making you nauseous in a pretty short amount of time.
Andria