1. Pharmaceuticals

• Antihistamine: Diphenhydramine HCl is widely used to relieve symptoms of allergy, hay fever, and the common cold such as sneezing, runny nose, itching, watery eyes, and throat irritation. It works by blocking histamine receptors (H1).
• Sedative and sleep aid: It is commonly used in over-the-counter sleep aids due to its drowsiness-inducing effects. It helps manage short-term insomnia and is included in many night-time cold and flu medications.
• Antiemetic (anti-nausea agent): Used to prevent and treat nausea, vomiting, and dizziness caused by motion sickness or inner ear disturbances. It helps by acting on the brain’s vomiting center and inner ear.
• Cough suppressant: Included in combination cold medications to help control cough associated with throat or airway irritation.
• Topical antipruritic (in creams or gels): Used in topical formulations to relieve itching from insect bites, minor skin irritations, and allergic skin reactions by reducing histamine activity locally.

2. Animal Healthcare

• Allergy relief and antiemetic in animals: Used under veterinary guidance to treat allergic reactions, itching, or motion sickness in pets, especially dogs and cats. The dosage and use are carefully controlled.

3. Personal Care 

• Soothing agent in medicated lotions: In low concentrations, it may be used in some medicated skin products to provide temporary relief from itching due to rashes, insect bites, or minor burns.