Precursor to metabolite: Mirtazapine to (Desmethyl-) mirtazapine

Mirtazapine is converted to (Desmethyl-) mirtazapine by CYP2D6 / CYP1A2.

The pair

Mirtazapine — Sedating, appetite-stimulating noradrenergic/serotonergic agent; known for weight gain.
(Desmethyl-) mirtazapine — Mirtazapine metabolite with the same H1 and 5-HT2 blockade at lower potency.

Anything that inhibits or induces CYP1A2, CYP2D6 shifts the balance between Mirtazapine and (Desmethyl-) mirtazapine.