Is mirtazapine an antihistamine?

Short answer:
Mirtazapine is not an antihistamine in the way diphenhydramine or cetirizine are, but it does block histamine H₁ receptors as part of its pharmacological profile. That antihistaminic activity is what gives it the sedating and weight‑gaining side‑effects people often notice.

What mirtazapine actually is

Antihistamine activity: what does it do?

• Sedation – H₁ blockade in the central nervous system (CNS) is the main reason mirtazapine can be used for sleep or as an adjunct for insomnia.
• Weight gain & increased appetite – H₁ antagonism can stimulate appetite and lead to modest weight changes in some patients.
• Dry mouth, constipation, blurred vision – Classic anticholinergic‑like side‑effects that overlap with H₁ blockade.

Because of these properties, mirtazapine is sometimes called a “non‑selective antihistamine” in pharmacology textbooks, but clinically it’s still marketed and prescribed as an antidepressant.

Practical take‑away

• If you’re looking for an antihistamine (for allergies, itching, or allergic rhinitis), mirtazapine is not the right choice. Over‑the‑counter H₁ blockers (loratadine, cetirizine, diphenhydramine, etc.) are far more appropriate.
• If you’re already taking mirtazapine for depression and notice sedation or weight changes, those side‑effects stem from its histamine‑blocking action, not from an “antihistamine prescription”.
• Side‑effect management – If the sedative effect is problematic, talk to your provider about dose timing or switching antidepressants. If weight gain is a concern, lifestyle modifications and possibly an alternative antidepressant may be considered.

Bottom line:
Mirtazapine does block H₁ histamine receptors (so it has antihistaminic properties), but it’s primarily an antidepressant, not an antihistamine medication.