Why do some people get flu like symptoms when quitting zoloft (sertraline)? what is happend to the body when the flu symptoms comes? is it the body who works hard?

Slower taper easier: "Flu" complaints are not uncommon. It depends on how quickly the dose is tapered. It is much more likely if the medication is stopped all at once. The timing of these symptoms depends on the half life of the medication.Short acting meds like paxil, (paroxetine) zoloft etc have more issues than long acting meds such as prozac. See link: http://www.webmd.com/depression/guide/withdrawal-from-antidepressant