Why do I feel worse the first week on an SSRI?

Because your brain notices the change before it benefits from it. Jitteriness, nausea, a bump in anxiety: all common in week one or two, and it usually fades as your body adjusts. The payoff runs the other direction, with most people feeling real improvement by week three or four.

Quitting in week one gets you the worst of the side effects and none of the benefit, which is the saddest trade in psychiatry. Call your prescriber if it's severe, if you feel agitated in a way that scares you, or if any thoughts of self-harm show up. That's a call-today situation, not a wait-for-your-follow-up one.