Originally Posted by
panic_down_under
Mirtazapine is mostly a very potent antihistamine at low doses blocking histamine H1 receptors which produces most of the sedation. It is also an antagonist (blocker) of serotonin 5-HT2a and 5-HT2c receptors which lengthens REM and slow brain sleep duration. At higher doses it enhances noradrenaline, aka norepinephrine, neurotransmission which is activating. The higher the dose the more activating it becomes.