Diphenhydramine, a brand name for a formulation containing essential Diphenhydramine, is widely used for various health benefits. This guide provides comprehensive information on the uses, dosage, side effects, and mechanism of action of Diphenhydramine, as well as insights into how long it takes to work. Understanding these aspects can help you make informed decisions about its use and effectiveness.


Diphenhydramine is a widely used over-the-counter antihistamine that interacts with a wide range of body systems, primarily the central nervous system. It is used to treat ailments such as allergies, motion sickness, insomnia, and some forms of arthritis.

Uses For

Diphenhydramine is used to treat a variety of ailments, including allergies, motion sickness, hay fever, insect stings, itching, insomnia, hives, sunburn, rashes, colds, and minor pain. It is also used to treat Parkinson’s disease, narcolepsy, and restless leg syndrome.

Mechanism Of Action

Diphenhydramine acts as a competitive antagonist of histamine H1 receptors, preventing histamine from binding to its receptor sites. This mechanism of action results in a decrease in the actions of histamine, including the symptom relief associated with its use.

How Long Does It Take to Work?

The effects of diphenhydramine can be felt anywhere from 30 minutes to 1 hour after ingestion. The peak of the effect is usually reached between 2 and 4 hours after ingestion.


Diphenhydramine is rapidly and completely absorbed orally, with peak concentrations of the drug in the blood occurring at approximately 1 hour after ingestion.

Route of Elimination

Diphenhydramine is rapidly metabolized and eliminated from the body, primarily in the urine. Its metabolites are primarily excreted through the urine as well.


The dosages of diphenhydramine vary depending on the indication for use and the route of administration. For oral administration: Adults: 25-50mg at bedtime or every 4-6 hours as needed Children age 6-12: 12.5-25mg at bedtime or every 4-6 hours as needed Children age 2-6: 6.25-12.5mg at bedtime or every 4-6 hours as needed For intramuscular administration: Adults: 25-50mg every 6-8 hours as needed For rectal administration: Adults: 25-50mg every 4-6 hours as needed


Diphenhydramine can be administered orally, intramuscularly, or rectally. Oral administration is the most common route of administration, followed by intramuscular injection and rectal administration.

Side Effect

Common side effects of diphenhydramine include dry mouth, drowsiness, dizziness, headache, nausea, vomiting, constipation, and blurred vision. Rare side effects may include confusion, chest pain, difficulty urinating, fever, irregular heartbeat, loss of coordination, muscle weakness, rash, seizures, and unusual excitement.


No cases of overdose resulting in death from diphenhydramine alone have been reported. Symptoms of overdose may include drowsiness, agitation, dilated pupils, dry skin, flushed face, hallucinations, nausea, rapid heartbeat, and vomiting.


Diphenhydramine should be used with caution in patients with glaucoma, asthma, coronary artery disease, hypertension, and high thyroid levels. It should also not be used in combination with other sedatives or alcohol.


Diphenhydramine may interact with other medications including sedatives, antacids, antibiotics, antifungals, thyroid medications, and medications used to treat depression or psychotic disorders.

Disease Interaction

Diphenhydramine may interact with several diseases, including asthma, glaucoma, coronary artery disease, hypertension, and hyperthyroidism. It should be used cautiously in patients with these conditions.

Drug Interaction

Diphenhydramine may interact with a variety of drugs, including sedatives, antacids, antibiotics, antifungals, thyroid medications, and medications used to treat depression or psychotic disorders. It should be used with caution and under the advice of a physician in combination with other drugs.

Food Interactions

Diphenhydramine may interact with certain foods, including coffee, tea, alcohol, chocolate, and some sharply flavored fruits and vegetables. These interactions can reduce the effectiveness of the drug or cause adverse side-effects.

Pregnancy Use

Diphenhydramine should not be used during pregnancy as it may cause harm to an unborn baby. It should be used only if prescribed by a physician and when the potential benefits outweigh the risks.

Lactation Use

Diphenhydramine should not be used during breastfeeding due to the potential for adverse effects on the newborn. However, it may be used if the potential benefits are deemed to outweigh the risks.

Acute Overdose

Signs of diphenhydramine overdose include drowsiness, agitation, dilated pupils, dry skin, flushed face, hallucinations, nausea, rapid heartbeat, and vomiting. Treatment for an overdose of diphenhydramine may include treatment with activated charcoal, stomach lavage, and supportive care.


Diphenhydramine should not be used in patients with hypersensitivity or allergy to the drug. In addition, it should not be administered to patients with narrow-angle glaucoma, severe respiratory, hepatic, or renal impairment, and angle-closure glaucoma.

Use Direction

Diphenhydramine should be taken exactly as prescribed by a physician or pharmacist. For oral administration, the drug should be taken with food or milk to avoid stomach upset. The drug should not be taken more frequently than prescribed or for longer duration of time than recommended.

Storage Condition

Diphenhydramine should be stored at room temperature, away from heat and light. It should also be kept out of reach of children and away from pets.

Volume Of Distribution

The apparent volume of distribution of diphenhydramine is 0.4 L/kg.


The elimination half-life of diphenhydramine is approximately 4-9 hours.


The clearance of diphenhydramine is 7.5 mL/minute/kg.

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