Zoloft contains sertraline which has a half life of about 1 day, that is if you just take one tablet after one day the level of sertraline in your system will be half. This means that the drug builds up in your body over time. That is why if you are taking a brand of sertraline tablets it will take a few weeks for in to have a impact on your mood as the levels rise and then even out.
When you stop taking the drug, however, it means that after 1 day you still have 50% of it in your system, day two 25% and day three 12.5%. If you follow the maths, after 7 days you will still have about 0.8% floating around your body. This means if you experience withdrawals it is not a matter of sweating it out for a day or two.
It is due to this half life effect and the serious withdrawal symptoms which can occur coming off Zoloft that Doctors recommend a gradual reduction in dose.