The first time I used it I had headaches non-stop and also found that it made me really "vague". I had a hard time concentrating on anything (to the point that I avoided driving because I thought I was unsafe on the road) and had trouble remembering words. I ended up reducing my work hours because I just wasn't able to be productive.

But the second and third times I didn't have the same s/e, just a mild headache and some fatigue.