Diphenhydramine is a first-generation antihistamine widely available over-the-counter (OTC) under various brand names, most notably Benadryl. Since its approval in 1946, it has been used to address a variety of health issues, from minor allergic reactions to occasional sleeplessness. Its versatility stems from its ability to block histamine, a chemical the body releases during allergic reactions, but also from its strong sedative and anticholinergic properties. However, these very properties mean that while effective, diphenhydramine is not suitable for everyone and has a different side effect profile than modern, second-generation antihistamines. Understanding its mechanism, uses, and risks is crucial for safe and effective use.
How Diphenhydramine Works in the Body
Diphenhydramine primarily works by blocking histamine from binding to H1 receptors throughout the body. Histamine is a natural compound involved in various bodily processes, including inflammatory responses that cause allergy symptoms like itching, sneezing, and a runny nose. By acting as an H1 receptor inverse agonist, diphenhydramine reverses the effects of histamine and provides relief from these symptoms.
What sets diphenhydramine apart from newer antihistamines is its ability to cross the blood-brain barrier. This allows it to act on H1 receptors in the central nervous system, which is what causes its most well-known side effect: drowsiness. This sedative property, often undesirable when treating allergies, is the reason it's also marketed as a sleep aid.
Beyond its antihistamine and sedative effects, diphenhydramine also has potent anticholinergic properties. This means it blocks the action of acetylcholine, another neurotransmitter, which can lead to side effects like dry mouth, blurred vision, and difficulty urinating. This anticholinergic activity also makes it useful for treating certain movement disorders and nausea associated with motion sickness.
The Primary Uses for Diphenhydramine
Relieving Allergy and Cold Symptoms
As an antihistamine, its most prominent use is for alleviating symptoms of allergic rhinitis (hay fever) and other types of allergies. This includes symptoms such as:
- Red, watery, or itchy eyes
- Sneezing
- Runny nose
- Itching and hives
It is also used to address some common cold symptoms, especially those caused by a histamine release, such as sneezing and a runny nose. However, it does not cure the cold itself. In severe cases of anaphylaxis, injectable diphenhydramine may be used as an adjunct to epinephrine, though it is not a first-line treatment.
Assisting with Occasional Insomnia
Because of its significant sedative effect, diphenhydramine is a common ingredient in many OTC sleep aids for treating occasional sleeplessness in adults and children 12 and older. It is important to note that it is not recommended for chronic insomnia and should not be used as a sleep aid for children under 12. The American Academy of Sleep Medicine has also recommended against its regular use for insomnia due to poor effectiveness over time and next-day grogginess.
Preventing and Treating Motion Sickness
Diphenhydramine's antiemetic properties make it useful for preventing and treating motion sickness. It works by blocking histamine receptors in the brain's vomiting center. For best results in preventing motion sickness, it should be taken about 30 minutes before travel.
Managing Movement Disorders
In some cases, diphenhydramine can be used to treat extrapyramidal symptoms and parkinsonism, which are movement abnormalities that can be a side effect of certain antipsychotic medications.
Important Side Effects and Precautions
Like any medication, diphenhydramine has potential side effects, many of which are related to its anticholinergic properties. Common side effects include:
- Drowsiness and dizziness
- Dry mouth, nose, and throat
- Thickened bronchial secretions
- Blurred vision
- Constipation
- Difficulty urinating
- Impaired coordination
More serious side effects can occur, especially with high or toxic doses, and warrant immediate medical attention:
- Severe dizziness or anxiety
- Hallucinations or confusion
- Seizures or tremors
- Vision problems, like seeing halos around lights
- Rapid or irregular heartbeat
- Difficulty breathing
Precautions must be taken for certain populations. The American Geriatrics Society considers it a high-risk medication for older adults due to an increased risk of falls, confusion, and memory issues. It is also generally avoided in children under 6 for cold symptoms and under 12 for sleep issues due to the risk of paradoxical excitement and serious side effects from overdose.
Patients should avoid consuming alcohol or taking other sedating medications while on diphenhydramine due to the compounded risk of sedation and impaired coordination. Furthermore, individuals with conditions like glaucoma, asthma, prostate disease, or difficulty urinating should consult a healthcare provider before use.
Diphenhydramine vs. Newer Antihistamines
To make an informed choice, it is helpful to compare diphenhydramine with modern, second-generation antihistamines.
| Feature | Diphenhydramine (Benadryl) | Cetirizine (Zyrtec) | Loratadine (Claritin) |
|---|---|---|---|
| Generation | First-generation | Second-generation | Second-generation |
| Drowsiness | High potential for drowsiness | Low to moderate potential for drowsiness | Low potential for drowsiness |
| Duration | Short-acting (4-6 hours) | Long-acting (24 hours) | Long-acting (24 hours) |
| Onset | Fast | Fast | Slow |
| Allergy Coverage | Effective for acute symptoms | Strong, fast relief | Effective for seasonal allergies |
| Cognitive Effects | Can cause cognitive impairment, especially in the elderly | Fewer cognitive side effects | Fewer cognitive side effects |
Conclusion
In summary, what is diphenhydramine for is a question with a few key answers: allergies, colds, insomnia, and motion sickness. Its effectiveness in these areas is well-established, largely due to its potent antihistamine and sedative effects. However, consumers must exercise caution due to the medication's significant potential for drowsiness and other anticholinergic side effects. Diphenhydramine should be used for short-term issues and is not recommended for chronic conditions like persistent insomnia. For many individuals, particularly children and older adults, safer alternatives with fewer side effects may be a better choice. Always consult a healthcare provider or pharmacist to determine if diphenhydramine is the most appropriate and safest option for your specific needs, especially if you take other medications or have underlying health conditions. For more detailed information, consider visiting the official MedlinePlus drug information page.
