An Overview of Diphenhydramine Hydrochloride
Diphenhydramine hydrochloride (DPH) is a first-generation antihistamine, a class of drugs that work by blocking the effects of histamine in the body. Histamine is a chemical released during an allergic reaction that causes symptoms such as sneezing, itching, and a runny nose. By blocking these effects, DPH provides relief from a wide range of conditions. Its availability over-the-counter (OTC) under numerous brand names, including Benadryl, Unisom, and ZzzQuil, makes it a common household medication. However, its sedative properties and other side effects mean it should be used with caution, especially in certain patient populations.
Primary Therapeutic Uses
Diphenhydramine is a multi-purpose medication with several key applications:
Relief for Allergy Symptoms
As an antihistamine, DPH is highly effective at relieving the symptoms associated with seasonal allergies, hay fever, and other allergic reactions. These include:
- Sneezing
- Runny nose
- Itchy, watery eyes
- Hives and other types of itching
Treating Common Cold Symptoms
Many of the same symptoms experienced during allergies can also occur with the common cold. DPH can help manage these symptoms, specifically:
- Sneezing
- Runny nose
Aiding Short-Term Insomnia
One of the most well-known side effects of DPH is drowsiness, which is why it is marketed as a sleep aid under brands like ZzzQuil and Unisom. It is intended for short-term use to help with occasional sleeplessness. The American Academy of Sleep Medicine, however, does not recommend its regular use for chronic insomnia. Tolerance can build quickly, making it less effective over time.
Preventing and Treating Motion Sickness
DPH can help prevent and treat the nausea, vomiting, and vertigo associated with motion sickness. It works by blocking histamine receptors in the vestibular system, which helps control balance. It is best to take the medication 30 to 60 minutes before exposure to motion.
Other Specific Medical Applications
Management of Movement Disorders
Due to its anticholinergic properties, DPH can be used to treat certain movement disorders. These include the symptomatic treatment of Parkinsonian syndrome and drug-induced extrapyramidal symptoms. This is typically done under the care of a healthcare professional.
Topical Itch Relief
Topical formulations, such as creams and gels, are available for external application to provide relief from itching caused by:
- Insect bites and stings
- Poison ivy, oak, and sumac
- Minor skin irritations and rashes
Topical use should be limited to small areas and is not recommended for widespread skin conditions.
How Diphenhydramine Works
DPH's primary mechanism of action is its role as an inverse agonist at the H1 histamine receptor. By binding to these receptors, it prevents histamine from exerting its effects, thereby reducing allergy symptoms. As a first-generation antihistamine, DPH readily crosses the blood-brain barrier and blocks H1 receptors in the central nervous system, leading to its sedating effect. This central nervous system activity also helps suppress the cough center in the brain.
Additionally, DPH has anticholinergic properties, meaning it blocks the action of acetylcholine. This can contribute to several side effects, such as dry mouth, blurred vision, and urinary retention. Finally, DPH can act as a sodium channel blocker, giving it local anesthetic properties.
Important Safety Considerations
Common Side Effects
- Drowsiness, which can affect concentration and reaction time
- Dry mouth, nose, and throat
- Dizziness
- Constipation
- Blurred vision
- Urinary retention or difficulty urinating
Special Population Warnings
- Children: DPH can cause paradoxical excitation (restlessness, agitation) in children. Oral formulations are not recommended for children under 2 for cold symptoms or under 12 for sleep.
- Older Adults: The American Geriatrics Society recommends avoiding DPH in individuals 65 and older due to strong anticholinergic effects, which increase the risk of falls, confusion, and memory issues. Long-term use in older adults may be associated with an increased risk of dementia.
- Pregnancy and Breastfeeding: Caution is advised for pregnant and breastfeeding women, and a healthcare provider should be consulted.
Diphenhydramine vs. Newer Antihistamines
| Feature | Diphenhydramine (Benadryl) | Second-Generation Antihistamines (e.g., Cetirizine/Zyrtec) |
|---|---|---|
| Mechanism | Competitively blocks H1 receptors; crosses the blood-brain barrier | Competitively blocks H1 receptors; minimally crosses the blood-brain barrier |
| Primary Effect | Reduces allergy symptoms; causes significant drowsiness | Reduces allergy symptoms; generally non-sedating or less-sedating |
| Speed of Onset | Works within 30 minutes, with peak effects in 1-2 hours | Generally comparable, with effects often within an hour |
| Duration of Action | Typically 4-6 hours | Effects last longer, usually 12-24 hours |
| Primary Use | Occasional allergies, temporary sleep aid, motion sickness | First-line treatment for chronic allergy symptoms |
| Long-Term Use | Not recommended due to tolerance, side effects, and risks, especially for older adults | Considered safer for long-term use in most adults |
When to Exercise Caution
Several factors can increase the risks associated with DPH use:
- Medical Conditions: Individuals with certain health issues should use DPH with caution, including those with glaucoma, enlarged prostate, urinary retention, severe heart disease, hypertension, and asthma.
- Drug Interactions: DPH has an additive effect with alcohol and other central nervous system (CNS) depressants, such as sedatives and opioids, increasing drowsiness and potential for adverse effects. It should not be taken with MAO inhibitors. Always check the label of combination products, like Tylenol PM or Advil PM, to avoid double-dosing.
- Overdose Risk: As an OTC medication, the risk of overdose can be overlooked. Taking too much DPH can lead to serious toxicity, including seizures, confusion, hallucinations, and cardiac arrhythmias. In case of a suspected overdose, contact poison control immediately.
Conclusion: Weighing the Benefits and Risks
Diphenhydramine hydrochloride is a versatile and accessible medication that can effectively treat a variety of symptoms, from allergic reactions and colds to temporary insomnia and motion sickness. However, its effectiveness is balanced by significant side effects, particularly drowsiness and anticholinergic effects, which make it less suitable for chronic use or for certain populations, including older adults and young children. For long-term allergy management, newer, non-sedating antihistamines are generally preferred. It is crucial to read labels, be aware of drug interactions, and consult a healthcare provider for personalized advice, especially if you have underlying medical conditions or are considering prolonged use. Informed and responsible use ensures that the benefits of DPH are realized while minimizing potential risks. For more detailed information on drug interactions, the Drugs.com Diphenhydramine page can be a useful resource.
