Why have I had a consistent headache on and off for about a week? What is the most likely cause of this, and is it anything life threatening?

Many possibilities. Most people have a headache at some time, such as from colds and flus, lack of sleep, too much driving, long hours on the computer, stress at school, etc... Several over-the-counter options include Tylenol, Motrin, Aleve, Excedrin, and their generic brands. A severe headache, recurrent or persistent headache (as in this case), or one with other worrisome symptoms should be checked by one's doctor.