Diphenhydramine, Structure, Mechanism of action, Drug Interactions, Therapeutic Uses, Adverse effects and Contraindications

Diphenhydramine

Diphenhydramine is an antihistamine medication that is commonly used to treat various allergic reactions, hay fever, and symptoms associated with the common cold.

Diphenhydramine is available under various brand names, including:

1. Benadryl: This is one of the most well-known brand names for diphenhydramine. Benadryl is available in various forms, including tablets, capsules, and liquid.

2. Nytol: Nytol is a brand that offers diphenhydramine as a sleep aid. It is commonly used to help individuals with occasional sleeplessness.

3. Sominex: Sominex is another brand that provides diphenhydramine as a sleep aid. It is designed to help individuals fall asleep and stay asleep throughout the night.

4. Unisom: Unisom is a brand that offers several sleep aids, and some of their products contain diphenhydramine as the active ingredient.

These are just a few examples of the brand names for diphenhydramine. 

Mechanism Of Action

The mechanism of action (MOA) of diphenhydramine involves its ability to antagonize or block the effects of histamine at the histamine H1 receptors. Histamine is a chemical that is released by the body during allergic reactions and contributes to the development of symptoms like itching, sneezing, runny nose, and watery eyes.

Diphenhydramine is a competitive antagonist of the histamine H1 receptors, which means it competes with histamine for binding to these receptors. By binding to the H1 receptors, diphenhydramine prevents histamine from exerting its effects. This blockade of histamine receptors leads to a reduction in the allergic response and alleviation of symptoms associated with allergies, such as itching, sneezing, and nasal congestion.

Additionally, diphenhydramine has anticholinergic properties, which means it can inhibit the action of the neurotransmitter acetylcholine. This contributes to some of the side effects of diphenhydramine, such as dry mouth, blurred vision, constipation, and urinary retention. The sedative effect of diphenhydramine is also attributed to its anticholinergic activity.

It's important to note that while diphenhydramine is primarily known for its antihistamine properties, its sedative effects and anticholinergic activity contribute to its use as a sleep aid and its potential side effects.

Drug Interactions

Diphenhydramine, like any other medication, can interact with other drugs, substances, or medical conditions. These interactions can alter the effectiveness or increase the risk of side effects. Here are some notable drug interactions involving diphenhydramine:

1. Central nervous system (CNS) depressants: Diphenhydramine has sedating properties, and combining it with other CNS depressants such as alcohol, benzodiazepines, opioids, or other sedating medications can increase the risk of excessive sedation, drowsiness, and respiratory depression.

2. Monoamine oxidase inhibitors (MAOIs): MAOIs are a class of antidepressant medications. Combining diphenhydramine with MAOIs can lead to severe side effects, including hypertensive crisis, which is characterized by a sudden and dangerous increase in blood pressure.

3. Anticholinergic drugs: Diphenhydramine has anticholinergic activity, and combining it with other medications that also possess anticholinergic properties can result in an additive effect, leading to increased risk of side effects such as dry mouth, constipation, urinary retention, blurred vision, and cognitive impairment.

4. Medications for overactive bladder: Combining diphenhydramine with drugs used to treat overactive bladder, such as antimuscarinic agents like oxybutynin or tolterodine, can increase the risk of urinary retention and other anticholinergic side effects.

5. Medications that cause drowsiness: Diphenhydramine can enhance the sedative effects of other medications that cause drowsiness, including certain antihistamines, sedatives, hypnotics, and tranquilizers. Combining these drugs can result in excessive sedation.

These are just a few examples of potential drug interactions involving diphenhydramine.

Therapeutic Uses

Diphenhydramine has several therapeutic uses, including:

1. Allergic reactions: Diphenhydramine is commonly used to relieve symptoms associated with allergic reactions, such as itching, sneezing, runny nose, watery eyes, and hives. It helps block the effects of histamine, a chemical released during an allergic response.

2. Seasonal allergies (hay fever): Diphenhydramine can be used to alleviate symptoms of seasonal allergies, including nasal congestion, sneezing, and itchy or watery eyes caused by exposure to allergens like pollen, mold, or pet dander.

3. Motion sickness: Diphenhydramine is sometimes used to prevent or treat motion sickness symptoms, such as nausea, vomiting, and dizziness, that can occur during travel by car, plane, or boat.

4. Insomnia: Due to its sedative properties, diphenhydramine is sometimes used as a short-term sleep aid for individuals experiencing difficulty falling asleep or staying asleep. It can help induce drowsiness and promote a more restful sleep.

5. Itching and skin conditions: Diphenhydramine can provide relief from itching associated with various skin conditions, including allergic reactions, insect bites, and rashes caused by hives or contact dermatitis.

It's important to note that while diphenhydramine can be effective for these conditions, it is essential to use it as directed and consult a healthcare professional or pharmacist before starting any new medication. 

Adverse Effects

While diphenhydramine can be beneficial for treating various conditions, it can also have adverse effects. Common adverse effects of diphenhydramine include:

1. Drowsiness and sedation: Diphenhydramine is known for its sedating properties, and drowsiness is a common side effect. This can impair your ability to drive or operate machinery safely.

2. Dry mouth and throat: Diphenhydramine can cause a dry mouth and throat, leading to discomfort and increased thirst.

3. Blurred vision: Diphenhydramine can cause temporary vision changes, such as blurred or double vision.

4. Urinary retention: Diphenhydramine can affect the muscles of the bladder, leading to difficulty in emptying the bladder completely. This can result in urinary retention.

5. Constipation: Diphenhydramine can cause constipation by slowing down bowel movements.

6. Dizziness and coordination problems: Some individuals may experience dizziness, unsteadiness, or problems with coordination while taking diphenhydramine.

7. Cognitive impairment: Diphenhydramine can have an impact on cognitive function and may cause confusion or difficulty with thinking and concentration, especially in older adults.

8. Paradoxical reactions: In rare cases, diphenhydramine can cause paradoxical reactions, including increased excitability, restlessness, or irritability.

Contraindications

Diphenhydramine has certain contraindications, which are situations or conditions where the use of the medication is not recommended. Here are some contraindications associated with diphenhydramine:

1. Hypersensitivity: Individuals who have a known hypersensitivity or allergy to diphenhydramine or other antihistamines should avoid using the medication.

2. Acute asthma attack: Diphenhydramine is generally contraindicated during an acute asthma attack, as it may not be effective in managing the condition and could potentially worsen respiratory symptoms.

3. Glaucoma: Diphenhydramine can increase intraocular pressure, which can be problematic for individuals with glaucoma. It is typically contraindicated in people with narrow-angle glaucoma.

4. Urinary retention: Diphenhydramine can impair bladder muscle function and cause urinary retention. It is contraindicated in individuals with urinary retention, such as those with bladder outlet obstruction or prostate enlargement.

5. Severe liver disease: Individuals with severe liver disease may have difficulty metabolizing diphenhydramine properly. Therefore, caution should be exercised, and the medication may be contraindicated in such cases.

6. Breastfeeding: Diphenhydramine can pass into breast milk and may affect the nursing infant. It is generally contraindicated or requires caution and medical supervision while breastfeeding.

7. Newborns and premature infants: Diphenhydramine is generally not recommended for use in newborns and premature infants due to their immature metabolic and elimination systems.