Mirtazapine

Tetracyclic NaSSA developed by Organon in the 1990s — α2 antagonist + 5-HT2A/2C/3 antagonist + potent H1 antagonist. A-tier evidence as antidepressant (top-3 efficacy ranking in Cipriani 2018 Lancet network meta-analysis), A-tier off-label evidence for depression-comorbid insomnia + appetite loss. Primary subjective profile is heavy sedation + carbohydrate cravings + +2-10 lb weight gain in first 3-6 months. For Dylan: SKIP-FOR-NOW with high confidence. The "features" of mirtazapine (sedation + weight gain + appetite stimulation) are bugs for an MMA athlete who needs to make weight, train at high intensity, sustain 6-12 hr cognitive output, and is already migrating his late chronotype earlier — every primary effect of this drug pulls in the wrong direction. Only flips to OPTIONAL-ADD if a clinical depression presents with comorbid weight loss + insomnia, where the same effects become therapeutic features.

Mirtazapine is the prototype NaSSA — Noradrenergic and Specific Serotonergic Antidepressant. The mechanism is unusual because it produces antidepressant effects without any serotonin or norepinephrine reuptake inhibition — it works entirely through receptor antagonism and disinhibition of monoamine release.

The α2 antagonist arm (the noradrenergic + serotonergic disinhibition leg):

• α2-adrenergic auto-receptor antagonism on noradrenergic neurons in locus coeruleus. Normally, α2 autoreceptors function as a "thermostat" — when NE levels rise in the synapse, α2 activation feeds back to silence the firing neuron. Blocking α2 cuts the brake → sustained NE release.
• α2-adrenergic hetero-receptor antagonism on serotonergic neurons in dorsal raphe. Same logic, different direction: NE normally inhibits 5-HT neuron firing through α2 hetero-receptors on serotonergic terminals. Blocking α2 hetero-receptors disinhibits 5-HT release.
• Net effect: simultaneous increase in synaptic NE + 5-HT — without blocking reuptake transporters (no SNRI-style activation, no SSRI-style sexual side effects, no discontinuation syndrome of the same class).
• Also α2A/α2B/α2C subtype-distinct effects matter at high doses (see "low-dose paradox" below).

The 5-HT receptor selectivity arm (the "specific serotonergic" leg — what the second S in NaSSA means):

• 5-HT2A antagonism: blocks the cortical 5-HT2A receptors that mediate SSRI-class side effects (anxiety, agitation, sexual dysfunction, insomnia). This is why mirtazapine doesn't have an SSRI side-effect profile despite raising serotonin.
• 5-HT2C antagonism: the appetite-relevant receptor. 5-HT2C activation normally suppresses appetite (this is part of how SSRIs and serotonergic stimulants curb hunger). Blocking 5-HT2C disinhibits feeding circuits + contributes to weight gain. Also disinhibits dopamine + norepinephrine in frontal cortex (same mechanism as agomelatine — selective DA/NE rise in PFC, not whole-brain stimulant DA dump).
• 5-HT3 antagonism: blocks the receptor that mediates SSRI-induced nausea + GI distress + headache. This is why mirtazapine is an unusually GI-clean antidepressant — and is sometimes used as an off-label antiemetic.
• Net serotonergic effect: raises synaptic 5-HT, then funnels that 5-HT exclusively through 5-HT1A receptors (which are not blocked) → 5-HT1A-mediated antidepressant + anxiolytic action without the 5-HT2/3-mediated side effect cluster.

The H1 antagonist arm (the sedation + weight gain leg):

• Mirtazapine is one of the most potent H1 inverse agonists in clinical use — Ki ~0.14 nM, comparable to or exceeding diphenhydramine (Benadryl) and doxepin. A single 15 mg dose produces >80% H1 receptor occupancy.
• H1 blockade is the dominant mechanism for: heavy sedation, sleep promotion, appetite stimulation (via hypothalamic histaminergic appetite-regulating circuits), weight gain, and a meaningful chunk of the next-day-grogginess profile.
• H1 antagonism is what makes mirtazapine subjectively feel like "diphenhydramine with antidepressant activity attached" at low doses — and it's the receptor that drives most of the disqualifying effects for an athlete.

The low-dose paradox (this is the counterintuitive-but-important detail):

• Lower doses (7.5-15 mg) are MORE sedating than higher doses (30-45 mg).
• Mechanism: at low doses (≤15 mg), H1 antagonism is the dominant pharmacology — the drug is functionally a potent antihistamine. At doses >30 mg, noradrenergic activation from α2 antagonism begins to offset H1 sedation — the rising NE tone produces alertness that partially counteracts the antihistamine fog.
• This is documented in the dosing literature (Pinerest poster: "Paradoxical Sedation with Lower Mirtazapine Dosing"; multiple psychopharmacology reviews).
• Practical implication: prescribers titrating from 15 mg → 30 mg → 45 mg often see less sedation at higher antidepressant doses, which is why mirtazapine's sedation is often described as a "low-dose-only" feature. But the appetite + weight effects do NOT show the same paradoxical fade — they persist or worsen at higher doses.

What mirtazapine does NOT do (clean negatives):

• No serotonin reuptake inhibition (no SSRI sexual side effects, no SSRI discontinuation syndrome of the same severity, less GI distress)
• No norepinephrine reuptake inhibition (no SNRI-style HR/BP activation profile)
• No dopamine reuptake inhibition (no stim-flavored agitation)
• No anticholinergic activity to speak of (despite tricyclic appearance) — no constipation, urinary retention, or anticholinergic cognitive dulling in the same league as TCAs
• No appreciable cardiotoxicity (clean ECG profile vs TCAs)
• No abuse liability (not scheduled, no dependence pattern, no recreational use signal)