Most of the time Nicotine = headache
Lower your nicotine or vape less, or at least use lower nic part of the time when
vaping a lot.
It helps me at home to leave my ecigs in another room.
2nd possible cause - dehydration - stay hydrated. Vaping can be drying.
3rd possible cause - detoxing off the 4000 chemicals in cigarettes - take vitamins, stay hydrated, eat and drink well
4th possible cause - you got auto batteries and are sucking too hard, or manual and are sucking too hard which isn't necessary at all on manuals, or using too thick juice in cartos and making them hard to drag too soon, or the wider tip of the ecig vs the end of a cigarette is bothering your jaw muscles (if using 510 or KR8 cartomizers, put a slim drip tip on the end instead of the endcap to get a smaller mouth size).
But most of the time, if you vape alot and you get a headache = nicotine
Nicotine is a vasoconstrictor. It can mess with your blood pressure and also cause rapid or irregular heartbeat. I never got headaches my whole life unless I had a sinus infection. Then I started vaping, and sure enough, every time I vaped excessively (even with 1 level lower nic) I get a headache in my right temple (and if I kept vaping too much I could get a couple of irregular heartbeats like I used to with cigs). So after the first 6 weeks I worked on lowering my nicotine and now I am at a level where I can vape a lot and not get a headache unless I really overdo it.