Mirtazapine — Remeron — is the antidepressant for the patient whose depression comes with insomnia, anxiety, and weight loss. The mechanism is unusual, the side effect profile is the opposite of bupropion's, and the right patient gets results other antidepressants don't deliver.
- Class
- Noradrenergic and Specific Serotonergic Antidepressant (NaSSA)
- Mechanism
- Alpha-2 adrenergic antagonist → disinhibits NE and 5-HT release; 5-HT2A/2C and 5-HT3 antagonism; strong H1 antagonism → sedation + appetite
- Typical dose
- 15-45 mg at bedtime
- Half-life
- ~20-40 hours
- FDA indications
- MDD
- Key adverse effects
- Sedation (more at lower doses, paradoxically), weight gain, dry mouth, constipation, increased cholesterol/triglycerides. Rare agranulocytosis.
Black box: Suicidal thinking/behavior in pediatric and young adult patients
Particularly useful for depression with insomnia, anxiety, or weight loss. Often added to SSRI/SNRI for sleep and to counteract sexual side effects ("California Rocket Fuel" = venlafaxine + mirtazapine). Low doses (7.5-15 mg) paradoxically more sedating than higher doses.
The mechanism is multi-receptor and worth understanding step by step. Mirtazapine is an antagonist at presynaptic alpha-2 adrenergic autoreceptors. Normally those autoreceptors sense norepinephrine in the cleft and slow further release — they're a brake. Blocking them disinhibits NE release. Because some serotonergic neurons receive alpha-2 input, blocking alpha-2 also indirectly increases serotonin release. So mirtazapine raises both NE and 5-HT without touching SERT or NET.
Then it blocks specific serotonin receptors. 5-HT2A blockade contributes to anxiety reduction and sleep. 5-HT2C blockade contributes to appetite increase. 5-HT3 blockade — the same mechanism as ondansetron — prevents nausea and may reduce sexual dysfunction. So mirtazapine raises serotonin generally but blocks the specific receptor subtypes that produce SSRI-typical side effects. The result is serotonergic antidepressant effect without sexual dysfunction or GI upset.
Finally, mirtazapine is a potent H1 antagonist. That's what causes the sedation and the appetite increase — both desired effects for the right patient, both unwanted for the wrong one.
Mirtazapine is the textbook example of dose-dependent receptor recruitment — its sedating and activating properties trade places as the dose climbs.
Mechanism note: The dose paradox — more sedating at LOW doses because H1 dominates until noradrenergic release overtakes it — is the single most important fact about mirtazapine. Dose UP, not down, if sedation is the problem.
Clinically, the target patient is unmistakable: depression with insomnia, anxiety, anorexia, weight loss. Mirtazapine helps the patient sleep, eases the anxiety, brings back appetite, and addresses the mood — all from one prescription. For the depressed patient who has lost ten pounds and can't sleep, this is the drug.
A counterintuitive dosing fact: mirtazapine is more sedating at lower doses than at higher. At 7.5 to 15 milligrams, the H1 antihistamine effect dominates — patients are heavily sedated. At 30 to 45 milligrams, the noradrenergic disinhibition becomes prominent and activates somewhat, partially offsetting the H1 effect. The patient who is over-sedated on 15 milligrams may do better at 30. The patient who is under-treated at 45 may not benefit further from going higher.
- Cost
- Generic: ~$5-15/month.
- Generic status
- Generic since 2004.
- Formulary typical
- Tier 1 generic. No PA.
- Access friction
- None. Counsel about weight gain at prescription — patients often surprised.
Prescriber tip: Cheap and well-tolerated. For depression + insomnia + anorexia, an underused first choice. Counterintuitive dosing (15 mg more sedating than 30 mg) confuses some patients — explain upfront.
Side effects: sedation (often desired), weight gain (often desired, but not always — counsel), dry mouth, modest lipid and triglyceride elevation, rare agranulocytosis. The agranulocytosis is uncommon but worth warning patients about — call if you develop fever, sore throat, or unexplained infection.
Mirtazapine combines well with SSRIs and SNRIs. The "California Rocket Fuel" combination — venlafaxine plus mirtazapine — is a well-known strategy for treatment-resistant depression. The mechanisms are complementary: SNRI on SERT and NET, mirtazapine on alpha-2 disinhibition and 5-HT receptor selectivity. The combined regimen produces broader serotonergic and noradrenergic effect than either alone.
For depression with insomnia and weight loss, mirtazapine is often the right answer.