I think its a common side effect of mirtazapine to feel drowsy, thats why its best to take it at night.
SSRIs like citalropam, fluxoetine, sertraline are activators... but can have the opposite effect overall, particularly if they disrupt your sleep.
I think they're safe as long as you listen to your doctors advice, you start on a low dose and increase gradually, and keep a diary of your side effects.