Everyone else answered this so well, I thought I'd leave it alone, but ....

A large part of nausea with lithium and Zoloft™ (sertraline) is due to what I call "start-up" side effects. Your body has to adjust to the electrochemical (via neurotransmitters) changes that the medications produce.

Basically, with both drugs you are increasing overall serotonin levels in the body as well as the brain. There are serotonin receptors (5-HT3, for the keeners like bptoo) in the gut (gastrointestinal tract) that are bombarded with serotonin, where there was little prior. The increased serotonin (due to blockade of presynaptic serotonin reuptake mechanism - ya, still with me beep?) is what causes nausea with Zoloft.

The lithium changes the reactivity of nerve cells (neurons) making them more resistant to firing (in a way, sorta) through changes in intracellular calcium ion concentrations (via blockade of the phosphoinosidyl pathway). Your body has to adjust to these changes, as well.

Within a month of steadily taking these meds, the nausea and "blah" feeling to subside (not go away - usually - but not nearly as bad as now).

I hope that this helps. - Cam