I was prescribed 18mg of Concerta and it was working pretty well. I am a 20 year old female about 110lbs, I was reading that 18mg is too low for an adult to use. I tried a 27mg dosage for a little while but I noticed I was having really bad heart palpitations to the point where my chest was in a lot of pain and increased anxiety problems, obsessive behavior etc.. so I went back down to 18mg and I felt better right away. I feel worried that this is a dosage that is too low for adults but I also know i am very sensitive to medications. My main question is why is my Concerta taking so long to kick in? I take it at 8am and I don't really feel the effects until after 10:00-11:00. Its frustrating because i have classes at university in the morning and i try to study before they begin and i cant seem to focus at all.