I have been prescribed Simvastatin on April 12, 2017, for cholesterol. I was prescribed Lipitor in 1996, but, had a severe reaction to it. So, I stopped taking it, changed my diet and the doctor, at that time, acted astonished that my cholesterol. I am concerned about this prescription for Simvastatin 20 mg. I m not sure what to expect and a little more afraid to take it now. How long should I take it, if after experiencing side effects (not being able to fall asleep is a major problem) should I alert my doctor?