Can quitting remeron (mirtazapine) cold turkey cause rapid weight loss? gained 30lbs on it.

Not necessarily: It is always easier to gain weight than to lose it. However, if it was the medicine that caused the weight gain then some weight loss should happen shortly after stopping it.

Dont stop meds alone: Many medications can cause wieght gain directly (increased appetite) & indirectly based on often less well understood pathways. Don't stop meds "cold turkey", especially if not under doctor's care. Suddenly stopping most medications can cause discontinuation syndrome (in some cases even precipitate dangerous withdrawal) and can lead to relapse or even recurrence...( & no automatic weight loss).