Okay, I'll chime in here. I have had my dse901b for just over 1 week and was getting headaches for the first 5 days or so. I was smoking low dosages (~7mg) so it was NOT an overdose. I should note that I am "cutting" 36mg liquid. I began cutting with pure PG but have since moved to half-half with VG and PG. I have also increased the dosage to ~11mg. Finally, I have adjusted the way I inhale, allowing it to be a smoother and more drawn-out process. All of this has helped and I now don't get any headaches at all and am totally infatuated with my 901.
Those saying to swith over to another model may well be wise. I have however adjusted my approach to suit the 901 since I've invested the money already, parts are cheap and readily available (when in stock- lol), and I like the amount of vapor it produces.
Good luck!