I was prescribed zoloft (sertraline) for moderate anxiety, had a panic attack after 4 days on it! why do ssris increase anxiety at first?

May not be drug: It is hard to tell if it's the drug or the anxiety you already have. When a SSRI is used for anxiety, it usually takes 4-8 weeks before you see improvement. The drug may not have had enough time. However, SSRIs in general are associated with an 11-percent rate of anxiety. It's not known whether it's caused by the drug or the condition. If you have more or worse attacks than usual, tell your MD.

Serotonin jitters: Serotonin jitteriness syndrome describes increased anxiety during the initiation of an antidepressant. The precise reason for this isn't known but it has been speculated that it is related to stimulation of serotonergic activity in the limbic cortex (limbic system is the emotional processing center of the brain). More research is needed to really understand how this works.