Diphenhydramine syrup is indicated for the treatment of various allergic conditions, including hay fever, allergic rhinitis, urticaria, angioedema, atopic dermatitis, contact dermatitis, gastrointestinal allergies, pruritus, physical allergies, reactions to contrast media injections, reactions to therapeutic preparations, and allergic transfusion reactions. It is also used to prevent and treat postoperative nausea and vomiting, motion sickness, certain forms of parkinsonism, and some types of allergic conjunctivitis. Additionally, it may be used occasionally as a nighttime sleep aid or for the short-term management of insomnia.

Diphenhydramine possesses antihistaminic (H1-receptor), antiemetic, anti-vertigo, sedative, and hypnotic properties. Its antihistamine effect works by blocking the spasmogenic and congestive actions of histamine, competing for H1 receptor sites on effector cells, thereby preventing—but not reversing—histamine-mediated responses. These receptors are located in the gut, uterus, large blood vessels, bronchial muscles, and other tissues. Its antiemetic effect occurs by inhibiting the medullary chemoreceptor trigger zone. The anti-vertigo effect is achieved through a central antimuscarinic action on the vestibular apparatus, the integrative vomiting center, and the medullary chemoreceptor trigger zone in the midbrain.

Antihistamine and antitussive:
Tablet:

• Adults: 25 mg to 50 mg 3 to 4 times daily.
• Children (10 years or over): 25 mg 3 or 4 times daily or as directed by the physician.

Syrup:

• Children under 2 years: 2.5 mL every 4 to 6 hours.
• Children 2 to under 6 years: 5 mL every 4 to 6 hours.
• Children 6 to 12 years: 10-20 mL every 4 to 6 hours.
• Adults and children≥12 years: 10-20 mL every 4 to 6 hours. 

Sleep aid: Adults and children over 12 years: 50 mg at bedtime.

Diphenhydramine may interact with certain antidepressants, especially tricyclic antidepressants and monoamine oxidase (MAO) inhibitors. MAO inhibitors can prolong and intensify the anticholinergic effects of antihistamines. Additionally, the central nervous system (CNS) effects of diphenhydramine may be enhanced by alcohol and other CNS depressant medications.

Diphenhydramine is contraindicated in premature or newborn infants and in individuals with hypersensitivity to any component of the drug. It should also be avoided in patients with asthma attacks, narrow-angle glaucoma, prostatic hypertrophy, stenosing peptic ulcer, pyloroduodenal obstruction, bladder neck obstruction, or those receiving antidepressant therapy.

Common side effects of diphenhydramine include drowsiness, dizziness, dry mouth, nausea, and vomiting. Less frequently, it may cause vertigo, palpitations, blurred vision, headache, restlessness, insomnia, and thickened bronchial secretions. Allergic reactions, diarrhea, vomiting, and excitation may also occur in some cases.

The safety of diphenhydramine during pregnancy and lactation has not been established. Therefore, its use in these patients should carefully weigh the potential benefits against possible risks.

Patients should be advised not to drive or operate heavy machinery until they know how the drug affects them.

Overdose of antihistamines can cause effects ranging from central nervous system depression to stimulation, with stimulation being more common in children. Anticholinergic (atropine-like) symptoms, such as dry mouth, fixed and dilated pupils, flushing, and gastrointestinal disturbances, may also occur.

Sedating Anti-histamine

Do not store above 30°C. Keep away from light and moisture, and out of reach of children.