Mirtazapine

Mechanism

Mirtazapine increases serotonin release by blocking presynaptic α2-adrenergic autoreceptors. Linezolid reversibly inhibits MAO. Combined action multiplies serotonergic activity.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia, dilated pupils. Autonomic features: profuse sweating, tachycardia, hypertension, fever above 38.5 °C. Severe cases progress to seizures, rhabdomyolysis, disseminated intravascular coagulation, and death. First signs appear within hours of concurrent dosing.

Management

The combination is not used: the FDA label for mirtazapine contraindicates it with MAO inhibitors, including linezolid, and within 14 days of stopping them. A planned linezolid course starts about 2 weeks after the last mirtazapine dose, the interval given in the mirtazapine SmPC. If linezolid is needed urgently, mirtazapine is stopped before the first antibiotic dose and the patient is watched for serotonin syndrome throughout the course. Mirtazapine is restarted no earlier than 14 days after the last linezolid dose. Trazodone is not a bridge: its FDA label also contraindicates linezolid. For gram-positive infections vancomycin or daptomycin are chosen, as they do not inhibit MAO.

Mechanism

Methylene blue (methylthioninium chloride) inhibits monoamine oxidase (MAO). The US Food and Drug Administration (FDA) mirtazapine label classes intravenous methylene blue as an MAO inhibitor, and the FDA methylene blue label cites literature describing it as a potent reversible inhibitor of this enzyme. Mirtazapine blocks presynaptic alpha-2 adrenergic receptors and enhances central noradrenergic and serotonergic transmission (mirtazapine labels). Together they can cause severe, sometimes fatal serotonin syndrome (methylene blue labels). No reports involved other routes of methylene blue, such as oral tablets or local tissue injection (FDA mirtazapine label).

Symptoms

Serotonin syndrome. Mental status changes: agitation, hallucinations, delirium, coma. Autonomic signs: tachycardia, labile blood pressure, dizziness, sweating, flushing, fever. Neuromuscular signs: tremor, rigidity, myoclonus, hyperreflexia, incoordination. Seizures, nausea, vomiting and diarrhoea may occur. Some reported cases were fatal (FDA methylene blue label).

Management

The combination is contraindicated: the US Food and Drug Administration (FDA) mirtazapine label contraindicates mirtazapine in patients taking MAO inhibitors, including intravenous methylene blue, or within 14 days of stopping them. The methylene blue labels approved by the European Medicines Agency (EMA) and the FDA name mirtazapine among the drugs to avoid with methylene blue. If a patient on mirtazapine needs methylene blue, stop mirtazapine before it is given (FDA mirtazapine label). When methylene blue cannot be avoided, give the lowest possible dose and observe the patient for nervous system effects for 4 hours (methylene blue labels). Monitor the ECG and blood pressure during and after the infusion (EMA methylene blue SmPC). If signs of serotonin syndrome appear, stop methylene blue and mirtazapine and start supportive treatment (FDA labels of both drugs). Restart mirtazapine no earlier than 14 days after the last methylene blue dose (FDA mirtazapine label).

Mechanism

Mirtazapine is a dual-mechanism antidepressant (α2 + 5-HT2 blockade). Combined with phenelzine causes serotonin syndrome.

Symptoms

Hyperthermia, myoclonus, tachycardia.

Management

Combination contraindicated. 2-week washout.

Mechanism

Both drugs depress the central nervous system. Mirtazapine increases the sedative effect of benzodiazepines (mirtazapine SmPC), and together they impair attention and coordination more than either drug alone (mirtazapine label, US Food and Drug Administration, FDA). The alprazolam SmPC warns that combining it with antidepressants deepens CNS depression and raises the risk of respiratory depression, coma and death (alprazolam SmPC).

Symptoms

Drowsiness, dizziness, slowed reactions, impaired attention and coordination (mirtazapine SmPC). In older or debilitated patients alprazolam more often causes ataxia and oversedation (alprazolam SmPC). Marked CNS depression can lead to respiratory depression (alprazolam SmPC).

Management

The combination is avoided where possible: the mirtazapine label approved by the US Food and Drug Administration (FDA) advises against taking it with benzodiazepines and names alprazolam. If both drugs are needed, the dose and duration of combined use are limited (alprazolam SmPC; alprazolam label, FDA). The alprazolam course is kept as short as possible, no longer than 2–4 weeks, and is not extended without reassessment (alprazolam SmPC). In older or debilitated patients alprazolam is started at 0.25 mg two or three times daily and kept at the lowest effective dose to avoid ataxia and oversedation (alprazolam SmPC). The patient is warned not to drive or operate machinery until it is clear how they tolerate the combination (mirtazapine SmPC; alprazolam label, FDA).

Mechanism

Mirtazapine prolongs QT dose-dependently. Combined with amiodarone, torsades de pointes risk rises, especially in older patients and with hypokalaemia.

Symptoms

QT prolongation on ECG. Clinically: dizziness, syncope, palpitations. Severe cases progress to polymorphic ventricular tachycardia (torsades de pointes) with risk of ventricular fibrillation and sudden cardiac death. Risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, and ischaemic heart disease.

Management

The combination is acceptable at low mirtazapine doses (≤30 mg/day) and normal potassium. ECG before start and at 2 weeks. Alternative antidepressant with minimal QT effect: agomelatine.

Mechanism

Mirtazapine lowers the seizure threshold less than bupropion, but the effect is additive. Both drugs may also cause insomnia and anxiety in combination.

Symptoms

Seizures (generalised tonic-clonic or focal), agitation, tremor, insomnia. Patients with prior epilepsy or eating disorders are at particular risk.

Management

The combination is possible for augmentation. Keep doses low: bupropion 300 mg, mirtazapine 30 mg/day. Avoid with prior epilepsy.

Mechanism

Mirtazapine acts on serotonergic transmission through a different mechanism, and its SmPC names SSRIs among the drugs that can bring on serotonin syndrome. Reports of QT prolongation and torsade with mirtazapine come mainly from overdose and from patients with other risk factors.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia, sweating, diarrhoea, tachycardia, and in severe cases hyperthermia and rigidity. Symptoms usually appear within a day of adding the second drug or raising the dose. In patients with cardiac risk factors QTc is tracked separately.

Management

The mirtazapine label does not forbid the combination. It asks for watching serotonergic symptoms in the first days after the start and after every dose increase. An ECG with QTc is for those who already carry cardiac risk factors.

Mechanism

Mirtazapine prolongs QT dose-dependently (especially above 30 mg/day). Combined with dronedarone, torsades de pointes risk rises.

Symptoms

QT prolongation on ECG. Dizziness, syncope, palpitations. Severe cases: polymorphic ventricular tachycardia (torsades de pointes). Risk is higher with hypokalaemia and hypomagnesaemia.

Management

The combination is acceptable at low mirtazapine doses (≤30 mg/day) and normal potassium. ECG before start and at 2 weeks. Alternative antidepressants with minimal QT effect: agomelatine or sertraline (up to 100 mg).

Mechanism

Mirtazapine + SNRI – augmentation in resistant depression ('California rocket fuel'). Serotonin syndrome is rare but possible at high doses.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The combination is possible for augmentation. Keep doses low: mirtazapine 15–30 mg, duloxetine 30–60 mg/day. Monitor blood pressure, pulse, and mental status in the early weeks.

Mechanism

Mirtazapine acts on serotonergic transmission through a different mechanism, and its SmPC names SSRIs among the drugs that can bring on serotonin syndrome. Reports of QT prolongation and torsade with mirtazapine come mainly from overdose and from patients with other risk factors.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia, sweating, diarrhoea, tachycardia, and in severe cases hyperthermia and rigidity. Symptoms usually appear within a day of adding the second drug or raising the dose. In patients with cardiac risk factors QTc is tracked separately.

Management

The mirtazapine label does not forbid the combination. It asks for watching serotonergic symptoms in the first days after the start and after every dose increase. An ECG with QTc is for those who already carry cardiac risk factors.

Mechanism

Additive serotonergic effect. Mirtazapine blocks 5-HT2/3 but activates 5-HT1A. Combined with a potent SSRI, serotonin syndrome has been reported, especially at high doses.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, fever.

Management

The combination is used for augmentation in resistant depression ('California rocket fuel'). Keep doses low: fluoxetine 20 mg, mirtazapine 30 mg/day. Monitor mental status in the early weeks.

Mechanism

Serotonergic load from combining two drugs with different mechanisms. Fluvoxamine also blocks CYP1A2 (a minor mirtazapine metabolic route); mirtazapine levels may rise.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, fever.

Management

The combination is used for augmentation in resistant depression ('California rocket fuel'). Keep doses low: mirtazapine 15–30 mg, fluvoxamine 100 mg/day. Monitor mental status in the early weeks.

Mechanism

Mirtazapine prolongs QT dose-dependently, additively with haloperidol (especially parenteral). Plus combined sedation.

Symptoms

QT prolongation on ECG. Dizziness, syncope, palpitations. Severe cases: polymorphic ventricular tachycardia (torsades de pointes). Risk is higher with hypokalaemia and hypomagnesaemia.

Management

Avoid the combination with mirtazapine above 30 mg and parenteral haloperidol. At low doses and normal potassium, the combination is acceptable with ECG monitoring at 2 weeks. Alternative antipsychotics with minimal QT effect: olanzapine or aripiprazole.

Mechanism

SSRI augmentation with mirtazapine in resistant depression is possible ('California rocket fuel'). Additive serotonergic effect is rare, but serotonin syndrome has been reported at high doses of both.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, fever.

Management

The combination is used for antidepressant augmentation. Keep doses low: sertraline 100 mg, mirtazapine 30 mg/day. Monitor blood pressure and mental status in the early weeks.

Mechanism

Tramadol is a serotonergic opioid; mirtazapine raises serotonergic transmission via α2-blockade. Additive serotonin syndrome risk.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, fever.

Management

If a patient taking mirtazapine needs tramadol, start with a low dose and keep the course short. Mirtazapine increases the sedative effect of opioids (mirtazapine SmPC), and the tramadol SmPC requires the lowest effective dose and shortest duration with CNS depressants. At the start and after each dose increase, watch for signs of serotonin syndrome and excessive drowsiness. Stop tramadol if serotonin syndrome is suspected. Both drugs lower the seizure threshold. In epilepsy, use tramadol only when it cannot be avoided; it is contraindicated in uncontrolled epilepsy (tramadol SmPC).

Mechanism

Additive sedation, anticholinergic effect, and weight gain. Mirtazapine's serotonergic effect (via α2-blockade) is not pronounced on a TCA.

Symptoms

Deep sedation, drowsiness, dry mouth, constipation, weight gain. Older patients: delirium and fall risk.

Management

Avoid the combination, especially in older patients. For resistant depression augmentation: mirtazapine + SSRI (sertraline, escitalopram) instead of a TCA. For neuropathic pain: gabapentin or pregabalin.

Mechanism

Additive QT prolongation (mirtazapine weakly prolongs QT, especially above 30 mg).

Symptoms

QT prolongation on ECG. Dizziness, syncope, palpitations. Rarely: polymorphic ventricular tachycardia (torsades de pointes). Risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, and ischaemic heart disease.

Management

For short azithromycin courses (3–5 days) with mirtazapine ≤30 mg, the combination is acceptable. In older patients and cardiovascular disease: ECG before start or alternative antibiotics (doxycycline, a cephalosporin).

Mechanism

Additive hypotension and sedation. Mirtazapine blocks α2-adrenergic and 5-HT2 receptors, causing orthostatic effects.

Symptoms

Lower blood pressure, postural dizziness, fatigue. Older patients: fall risk.

Management

The combination is acceptable at low mirtazapine doses (15–30 mg). Warn the patient about drowsiness and orthostatic dizziness, especially in the first 1–2 weeks. Check blood pressure standing and sitting.

Mechanism

Both drugs enhance serotonergic transmission. Venlafaxine inhibits serotonin and noradrenaline reuptake; mirtazapine blocks presynaptic α2 receptors and increases noradrenergic and serotonergic transmission. Together they raise the risk of serotonin syndrome (both SmPCs). Post-marketing reports of QT prolongation and torsades de pointes exist for both drugs, mostly in overdose or with other risk factors. In a dedicated QT study, mirtazapine 45 and 75 mg prolonged QT by an amount not considered clinically meaningful (FDA mirtazapine label).

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, fever. Signs of arrhythmia: palpitations, skipped beats, fainting.

Management

Mirtazapine is added to venlafaxine on purpose when depression has not responded, or has responded only partly, to an antidepressant. NICE allows such a combination of drugs from different classes, but advises warning the patient about a possible higher side-effect burden and considering a specialist consultation. Observe the patient closely at the start of co-treatment and after each dose increase (both SmPCs). If signs of serotonin syndrome appear, stop both drugs at once and start symptomatic treatment (FDA labels of both drugs). Before starting venlafaxine, measure blood pressure and bring existing hypertension under control; then check blood pressure regularly and after each dose increase (venlafaxine labels). In patients at high risk of serious arrhythmia or QT prolongation, weigh benefit and risk before prescribing venlafaxine (venlafaxine SmPC). In known cardiovascular disease or a family history of QT prolongation, prescribe mirtazapine with caution (mirtazapine labels).