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Pharmacology · Drugs and Prescriptions

Diphenhydramine

Dimedrolum

Diphenhydramine is a first-generation antihistamine with pronounced sedative and M-anticholinergic effects. The drug is widely used in allergology, anesthesiology, and neurology due to its ability to cross the blood-brain barrier.

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Pharmacological groupFirst-generation H1-histamine receptor blocker
Mechanism of actionCompetitive blockade of H1-receptors and central muscarinic (M) receptors
Dosage formsTablets, powder (0.02–0.05 g), suppositories, 1% solution (1 ml)
Routes of administrationEnteral (oral) and parenteral (IM, IV)
How to write a prescription
Rp.: Tab. Dimedroli 0,05
D.t.d. N 20
S. Внутрь по 1 таблетке 2 раза в сутки (на ночь)

Other spelling variants: Tabulettas Dimedroli 0.05; Tab. Dimedroli 0.03; Sol. Dimedroli 1% - 1 ml

Dosage form
Tab., powder 0.02, 0.03, and 0.05 g; suppositories 0.005, 0.01, 0.015, and 0.02 g; amp. solution 1% - 1 ml
Administration
Orally 0.03–0.05 g 1–3 times daily (maximum single dose 0.1 g; maximum daily dose 0.25 g); IM (maximum single dose 0.05 g; maximum daily dose 0.15 g)
Group
Agents affecting immune processes and suppressing inflammation
3 min readFor medical studentsUpdated 09/24/2026
Material prepared by WhiteCoat Pro editorial team based on platform study guidesReviewed by medical editor: Aulov A. A., general practitioner · 09/24/2026

Mechanism of action

The drug is a competitive antagonist of H1-histamine receptors. By blocking them, it prevents the effects of histamine: it blocks the calcium cascade initiation, smooth muscle contraction, and vascular permeability increase. A key feature is high lipophilicity, which allows diphenhydramine molecules to easily cross the blood-brain barrier (BBB), causing central nervous system effects. Additionally, the drug exhibits M-anticholinergic properties and local anesthetic activity via sodium channel blockade.

Pharmacological effects

  1. Antihistamine: suppression of allergic reactions.
  2. Sedative and hypnotic: central nervous system depression due to brain H1-receptor blockade.
  3. Antiemetic and anti-motion sickness: effective in motion sickness (kinetosis).
  4. Antiparkinsonian: management of extrapyramidal symptoms induced by neuroleptics.
  5. Local anesthetic: ability to block sodium channels in nerve fibers.
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Indications

The drug is prescribed for allergic conditions (urticaria, pruritus, allergic rhinitis, angioedema). In clinical practice, it is used as a sedative, a premedication component (e.g., during administration of muromonab-CD3 to prevent 'cytokine release syndrome'), a treatment for extrapyramidal side effects of antipsychotics, and a remedy for motion sickness.

Side effects and limitations

The main side effects are drowsiness, dry mouth (due to M-anticholinergic action), and potential gastrointestinal disturbances. It is important to note that in bronchial asthma and anaphylactic shock, diphenhydramine is not the drug of choice. Due to conflicting safety data, the drug is not recommended for pregnant women.

Prescription details

Orally, 0.03–0.05 g 1–3 times a day. Maximum single doses (MSD) for oral administration are 0.1 g, maximum daily doses (MDD) are 0.25 g. For intramuscular administration, the MSD is 0.05 g, and the MDD is 0.15 g. The drug requires caution when performing tasks that require concentration.

Briefly: answers to topical questions

What side effects does diphenhydramine cause?

Diphenhydramine causes the following side effects:

  • Hypnotic effect — pronounced central nervous system depression; often considered a side effect, but also used therapeutically.
  • M-anticholinergic action — pronounced atropine-like effect.

With which drug classes does diphenhydramine show synergism?

Sources confirm the co-administration of diphenhydramine:

  • With antipsychotics (neuroleptics) — to enhance sedation and reduce neurological side effects, including extrapyramidal symptoms.
  • With chlorpromazine, analgin, and promedol — as part of a neuroplegic lytic cocktail providing combined anticonvulsant, antihistamine, and sedative effects.
  • With interferon alfa-2a — in eye drops; combines antiviral, anti-inflammatory, and local anesthetic effects.

What are the contraindications to prescribing diphenhydramine?

Contraindications and limitations indicated in the sources include:

  • Pregnancy — H1-receptor blockers are not prescribed due to conflicting safety data.
  • Delirium — diphenhydramine is contraindicated due to its anticholinergic action.
How to memorize

Dimedrol — 'Dime-drowsy' (causes drowsiness) and 'Dime-dry' (dry mouth due to anticholinergic blockade).

Class and exam questions

Why does diphenhydramine cause drowsiness?

The drug crosses the blood-brain barrier and blocks H1-histamine receptors in the central nervous system, producing a depressant effect on the CNS.

Is diphenhydramine the drug of choice for anaphylactic shock?

No, in anaphylactic shock, antihistamines are not the drugs of choice; the primary role belongs to adrenomimetics (epinephrine).

What role does diphenhydramine play in neuroleptic therapy?

It is used to treat extrapyramidal symptoms arising as a side effect of antipsychotic therapy, owing to its antiparkinsonian action.

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What else to study on this topic

  • Comparative characteristics of first- and second-generation H1-blockers.
  • Mechanism of the 'cytokine storm' development and the role of premedication.
  • Effect of M-anticholinergic activity on smooth muscle.
  • Pharmacokinetic differences between lipophilic and hydrophilic antihistamines.
  • Rationale for avoiding antihistamines in hyperemesis gravidarum.

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