The reason you start 5–7 days (at least) before you quit is because it takes some time for the medicine to build up in the blood. Over about a week, your blood levels will rise reaching a “steady state.” This is a constant level of medication high enough to work. Trying to quit before that time decreases your chances of quitting as you will basically be trying to quit cold turkey without any help. Bupropion is continued for 12 weeks because 12 weeks of abstinence makes long-term quitting success much more likely.
Also, each dose should be taken at least 8 hours apart. Please, do not take more than 2 in a day. If you miss a dose, take the medication as soon as you remember, but not if it is close to the time you would be taking your next scheduled dose—and never take “extra” medicine to make up for a missed dose.
This information is not, nor is it intended to be, a substitute for professional medical advice, diagnosis, or treatment, and should never be relied upon for specific medical advice. If you have any questions or concerns, please talk to your doctor.