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Diphenhydramine

Q: What adverse effects are associated with diphenhydramine?

Diphenhydramine causes the following adverse effects: Hypnotic/Sedative effect — pronounced central nervous system depression; often considered an adverse effect, but utilized therapeutically as well.Antimuscarinic action — classic atropine-like anticholinergic side effects (dry mouth, blurred vision, urinary retention, constipation).

Diphenhydraminum

For medical students2 min readUpdated 2026-10-10

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

Pharmacological classFirst-generation H1 receptor antagonist

Mechanism of actionCompetitive blockade of H1 receptors and central muscarinic (M) receptors

FormulationsTablets, powder, suppositories, 1% solution

Routes of administrationEnteral (oral) and parenteral (IM, IV)

Mechanism of Action

The drug acts as a competitive antagonist of H1 histamine receptors. By blocking these receptors, it prevents the cellular effects of histamine: inhibiting calcium signaling, smooth muscle contraction, and vascular permeability. A key characteristic is its high lipophilicity, allowing diphenhydramine molecules to easily cross the blood-brain barrier (BBB), which accounts for its central nervous system effects. Additionally, the drug exhibits antimuscarinic properties and local anesthetic activity via the blockade of neuronal voltage-gated sodium channels.

Pharmacological Effects

Antihistamine: Suppression of acute allergic reactions.
Sedative and hypnotic: Central nervous system depression due to the blockade of brain H1 receptors.
Antiemetic and anti-vertigo: Effective against motion sickness (kinetosis).
Antiparkinsonian: Amelioration of extrapyramidal symptoms induced by neuroleptics (antipsychotics).
Local anesthetic: Ability to block sodium channels in nerve fibers.

Indications

The drug is indicated for allergic conditions (urticaria, pruritus, allergic rhinitis, angioedema). In clinical practice, it is used as a sedative, a component of premedication (e.g., prior to administration of muromonab-CD3 to prevent cytokine release syndrome), an agent to manage extrapyramidal side effects of antipsychotics, and for the management of motion sickness.

Adverse Effects and Limitations

Principal adverse effects include somnolence, xerostomia (dry mouth due to antimuscarinic action), and potential gastrointestinal disturbances. It is crucial to note that diphenhydramine is not the drug of choice for acute asthma exacerbations or anaphylactic shock. Due to conflicting safety data, the drug is generally not recommended during pregnancy.

Administration and Dosing

Orally, it is administered at 0.03–0.05 g 1–3 times daily. Maximum single and daily doses apply based on the route of administration. Intramuscular administration requires careful dosing. The drug requires caution in patients performing tasks that require mental alertness and motor coordination.

Mnemonic

Diphenhydramine — "Drowsy-ramine" (causes sedation) and "Dry-ramine" (dry mouth due to antimuscarinic blockade).

Frequently asked questions

What adverse effects are associated with diphenhydramine?

Diphenhydramine causes the following adverse effects:

Hypnotic/Sedative effect — pronounced central nervous system depression; often considered an adverse effect, but utilized therapeutically as well.
Antimuscarinic action — classic atropine-like anticholinergic side effects (dry mouth, blurred vision, urinary retention, constipation).

With which drug classes does diphenhydramine exhibit synergism?

Clinical literature confirms the combined use of diphenhydramine:

With antipsychotics (neuroleptics) — to potentiate sedation and mitigate neurological side effects, including extrapyramidal symptoms.
With chlorpromazine, analgin, and promethazine — as part of a neurolytic lytic cocktail providing combined sedative, antihistamine, and autonomic-stabilizing actions.
With interferon alfa-2a — in ophthalmic formulations combining antiviral, anti-inflammatory, and local anesthetic effects.

What are the contraindications to diphenhydramine administration?

Standard clinical contraindications and limitations include:

Pregnancy — first-generation H1 antihistamines should be avoided or used with caution due to conflicting safety profiles.
Delirium — diphenhydramine is contraindicated due to its central anticholinergic activity, which can worsen confusional states.

Why does diphenhydramine cause drowsiness?

The drug crosses the blood-brain barrier and blocks central H1 histamine receptors, leading to central nervous system depression.

Is diphenhydramine the drug of choice for anaphylactic shock?

No, antihistamines are not drugs of choice for anaphylactic shock; the primary lifesaving intervention is epinephrine (an adrenomimetic).

What role does diphenhydramine play in antipsychotic therapy?

It is used to treat extrapyramidal symptoms arising as adverse effects of antipsychotic therapy, leveraging its central anticholinergic/antiparkinsonian properties.

Go deeper

Comparative pharmacology of first- and second-generation H1 antihistamines.
Mechanism of cytokine release syndrome and the rationale for premedication.
Impact of antimuscarinic activity on smooth muscle and autonomic tone.
Pharmacokinetic differences between lipophilic and hydrophilic antihistamines.
Rationale against relying on antihistamines for hyperemesis gravidarum.