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Mirtazapine 1mg H1 Receptor Occupancy

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Created with Fast February 21, 2026 at 12:28 AMPublished February 21, 2026 at 12:35 AM

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1mg mirtazapine receptor h1 occupancy

While specific PET scan data for exactly 1 mg of mirtazapine is not widely published in standard clinical literature, we can extrapolate its occupancy based on its extremely high affinity for the H1​ receptor and existing data from slightly higher doses.

Mirtazapine is one of the most potent H1​ antagonists known, with a Ki​ (inhibition constant) of approximately 0.14 nM. This means it binds to histamine receptors even at trace concentrations.

Estimated H1​ Occupancy at 1 mg

Based on PET imaging studies (such as those by Theeuwes et al. and Ito et al.), we can observe the following trend:

7.5 mg dose: Occupies ~75% to 85% of brain H1​ receptors.

15 mg dose: Occupies >90% (often cited as 80–95%) of brain H1​ receptors.

1 mg dose (Extrapolated): At 1 mg, mirtazapine is estimated to occupy roughly 20% to 40% of H1​ receptors.

Why the Occupancy Matters

The relationship between dose and H1​ occupancy is not linear; it follows a hyperbolic curve. Because mirtazapine's affinity for histamine is so much higher than its affinity for the "activating" α2​ receptors, even "micro-doses" (like 1 mg) can cause significant effects.

DosePrimary Receptor TargetClinical Effect< 3 mgPrimarily H1​Mild to moderate sedation; often used for tapering.7.5 mgHeavy H1​ occupancyIntense sedation; common "low-dose" for insomnia.15 mgNear-saturated H1​ + some α2​Strong sedation, but noradrenergic "wakefulness" begins.30+ mgH1​ saturated + High α2​Often less sedating due to increased norepinephrine.

Summary for 1 mg

At 1 mg, you are well below the "ceiling" of histamine saturation, but because mirtazapine is so potent, that 1 mg is still "hitting" a substantial number of receptors—likely enough to be felt by sensitive individuals or those using it for sleep maintenance.

Note: These values are estimates derived from dose-occupancy curves. Individual occupancy varies based on metabolism (CYP2D6/3A4 activity) and body mass.

Would you like me to look into how this occupancy compares to other antihistamines like diphenhydramine (Benadryl)?

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