Histergan (Diphenhydramine Hydrochloride)

Histergan is indicated for several common allergic and related conditions.

• Relief of nasal congestion, sneezing, and itching caused by allergic rhinitis.
• Management of hives (urticaria) and skin itching.
• Temporary alleviation of cough, sore throat, and other minor symptoms of the common cold.
• Short‑term treatment of occasional insomnia when a non‑prescription sleep aid is desired.

• Diphenhydramine hydrochloride acts as a competitive antagonist at peripheral histamine H1 receptors, preventing histamine‑mediated vasodilation, increased vascular permeability, and sensory nerve stimulation. The drug also exhibits anticholinergic activity by blocking muscarinic receptors, which contributes to its sedative properties. By reducing central histaminergic neurotransmission, it produces drowsiness that can aid short‑term sleep, while peripheral blockade alleviates allergic symptoms and improves patient comfort.

Like other first‑generation antihistamines, Histergan may cause a range of unwanted effects, most of which are mild and transient.

Common

• Drowsiness or sedation.
• Dry mouth, throat, or nasal passages.
• Dizziness or light‑headedness.
• Nausea or upset stomach.

Serious

• Severe allergic reaction (anaphylaxis) with rash, swelling, or difficulty breathing.
• Rapid or irregular heartbeat (arrhythmia).

Before using Histergan, consider the following precautions to minimize risk and ensure safe use.

• Avoid use if you have known hypersensitivity to diphenhydramine or other first‑generation antihistamines.
• Use caution when driving, operating machinery, or performing tasks that require alertness due to possible sedation.
• Do not combine with alcohol or other central nervous system depressants.
• Consult a healthcare professional before use during pregnancy, breastfeeding, or in children under six years of age.
• Patients with glaucoma, enlarged prostate, urinary retention, or certain heart conditions should discuss use with a clinician.
• Limit use to the shortest duration needed for symptom relief.

Histergan can interact with several medicines; some combinations should be avoided while others require careful monitoring.

Avoid

• Alcohol and other sedating agents (e.g., benzodiazepines, opioids) because additive CNS depression can occur.
• Monoamine oxidase inhibitors (MAOIs) or recent MAOI therapy, which may increase anticholinergic effects.

Use with caution

• Other antihistamines or cold remedies containing diphenhydramine, leading to excess dosing.
• Anticholinergic drugs (e.g., tricyclic antidepressants, antipsychotics) may enhance dry‑mouth or urinary retention.
• Certain antidepressants (e.g., SSRIs) that can increase risk of serotonin syndrome when combined with high doses.
• Medications that prolong the QT interval, such as some antiarrhythmics, should be reviewed.