Depending on the half-life of your specific antidepressant, you might begin to feel sick after just a few hours if you miss a dose. Symptoms vary with the antidepressant and your usual dose, and they run the gamut from irritability and anxiety to fatigue, nausea, and unusual dreams. If you've missed your dose by just a few hours, take it as soon as you remember. If it's closer to the time you'd take it tomorrow, take it then instead.