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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice.
Diphenhydramine Hydrochloride
Brand Name
Allergy Relief Caplet
Generic Name
Diphenhydramine Hydrochloride
Active Ingredient
DiphenhydramineCategory
Nonsteroidal Anti-inflammatory Drug [EPC]
Salt Form
Hydrochloride
Variants
1
Different strengths and dosage forms
| Strength | Form | Route | NDC |
|---|---|---|---|
| 25 mg/1 | TABLET, FILM COATED | ORAL | 36800-937 |
Detailed information about Allergy Relief Caplet
References used for this content
This page is for informational purposes only and does not replace medical advice. Before using any prescription or over-the-counter medication for Allergy Relief Caplet, you must consult a qualified healthcare professional.
Diphenhydramine is a first-generation antihistamine and H1 receptor antagonist used to treat allergic reactions, insomnia, and motion sickness. It is known for its potent sedative and anticholinergic properties.
For the treatment of allergic symptoms or motion sickness, the standard adult dosage typically ranges from 25 mg to 50 mg taken every 4 to 6 hours. The maximum daily dose should generally not exceed 300 mg in a 24-hour period. For use as a nighttime sleep aid, a single dose of 50 mg is commonly taken 30 minutes before bedtime.
Pediatric dosing is strictly based on age and weight. For children aged 6 to 12 years, the typical dose is 12.5 mg to 25 mg every 4 to 6 hours, not to exceed 150 mg daily. For children under 6 years of age, diphenhydramine should only be used under the direct supervision of a pediatrician, as the risk of toxicity is higher in this population.
In patients with significant kidney disease, the interval between doses may need to be extended. Since the drug is primarily excreted as metabolites, clinical monitoring for increased sedation is recommended.
Because diphenhydramine is extensively metabolized by the liver, patients with hepatic dysfunction (liver failure) may experience prolonged drug effects. Lower doses or increased dosing intervals may be necessary.
Older adults (65+) are more sensitive to the anticholinergic effects of diphenhydramine. Healthcare providers often recommend lower starting doses to minimize the risk of confusion, dizziness, and falls.
Diphenhydramine can be taken with or without food. Taking it with food or milk may help reduce potential stomach upset. If using the liquid form, always use a calibrated measuring device (not a household spoon) to ensure an accurate dose. For motion sickness, the first dose should be taken 30 to 60 minutes before the activity that causes the condition. Store the medication at room temperature, away from moisture and direct heat.
If you miss a dose, take it as soon as you remember. However, if it is nearly time for your next scheduled dose, skip the missed dose and resume your regular schedule. Do not double the dose to catch up, as this increases the risk of adverse effects.
Signs of a diphenhydramine overdose may include extreme drowsiness, dilated pupils, fever, flushing, agitation, hallucinations, and in severe cases, seizures or cardiac arrhythmias. In the event of a suspected overdose, contact your local poison control center or seek emergency medical attention immediately.
> Important: Follow your healthcare provider's dosing instructions. Do not adjust your dose without medical guidance.
The most frequent side effect of diphenhydramine is somnolence (marked drowsiness). Patients often describe a 'foggy' feeling or decreased alertness that can last several hours. Other common effects include xerostomia (dry mouth), dry nose, and dry throat, which result from the drug's anticholinergic activity. These symptoms typically resolve once the medication is discontinued.
> Warning: Stop taking Diphenhydramine and call your doctor immediately if you experience any of these.
Prolonged use of diphenhydramine, especially in higher doses, may lead to the development of tolerance, where the body requires more of the drug to achieve the same effect. There is also emerging clinical evidence suggesting a potential link between the long-term, daily use of anticholinergic drugs and an increased risk of cognitive decline or dementia in older populations.
No FDA black box warnings currently exist for Diphenhydramine. However, the FDA has issued safety communications regarding the 'Benadryl Challenge' on social media, warning that higher than recommended doses can lead to serious heart problems, seizures, coma, or death.
Report any unusual symptoms to your healthcare provider.
Diphenhydramine is a central nervous system (CNS) depressant. Patients should be aware that its effects are cumulative when combined with other sedating substances. It should be used with extreme caution in patients with conditions that may be exacerbated by anticholinergic activity, such as asthma or cardiovascular disease.
No FDA black box warnings for Diphenhydramine.
While routine lab monitoring is not typically required for short-term OTC use, patients on long-term or high-dose prescription diphenhydramine may require periodic assessments of:
Diphenhydramine significantly impairs the mental and physical abilities required for driving or operating heavy machinery. The impairment can persist even the morning after taking the drug as a sleep aid. Do not engage in hazardous activities until you know how this medication affects you.
Alcohol consumption must be avoided while taking diphenhydramine. Alcohol synergistically increases the sedative effects of the drug, leading to dangerous levels of respiratory depression and impaired motor control.
For short-term use, diphenhydramine can typically be stopped without tapering. However, if used chronically for sleep, sudden discontinuation may result in 'rebound insomnia,' where sleep difficulties temporarily worsen. Discuss a cessation plan with your doctor if you have been using it daily.
> Important: Discuss all your medical conditions with your healthcare provider before starting Diphenhydramine.
There are no major contraindications regarding specific foods; however, taking the medication with a high-fat meal may slightly delay the time it takes to reach peak concentration (Tmax), though it does not typically reduce the overall absorption.
Diphenhydramine may interfere with allergy skin tests. Patients should typically stop taking antihistamines at least 4 days before such testing to avoid false-negative results. It may also occasionally cause false-positive results in urine drug screens for methadone or certain tricyclic antidepressants.
> Important: Tell your doctor about ALL medications, supplements, and herbal products you are taking.
Patients who are allergic to other ethanolamine-derivative antihistamines (such as clemastine or dimenhydrinate) may also experience allergic reactions to diphenhydramine. If you have had a reaction to an antihistamine in the past, inform your healthcare provider before use.
> Important: Your healthcare provider will evaluate your complete medical history before prescribing Diphenhydramine.
Diphenhydramine is historically classified as FDA Pregnancy Category B. This means animal reproduction studies have failed to demonstrate a risk to the fetus, but there are no adequate and well-controlled studies in pregnant women. While it is often used for short-term relief of severe nausea or allergies during pregnancy, it should only be used if clearly needed and under medical supervision, particularly during the third trimester, as it may cause uterine contractions.
Diphenhydramine is excreted in breast milk. Exposure to the infant can cause unusual excitement or irritability. Additionally, because antihistamines can decrease serum prolactin levels, they may interfere with the mother's milk supply. Healthcare providers generally recommend alternative treatments for breastfeeding women.
Diphenhydramine is approved for use in children 6 years and older. In younger children, it can cause paradoxical excitation (hyperactivity) rather than sedation. There have been reports of fatalities in infants and toddlers due to overdosage in multi-symptom cold products. Never use diphenhydramine to make a child sleepy.
According to the American Geriatrics Society (2023) Beers Criteria, diphenhydramine is considered a 'potentially inappropriate medication' for the elderly. Older adults have a higher risk of dizziness, sedation, and hypotension, which significantly increases the risk of falls and hip fractures. It may also worsen pre-existing cognitive impairment or dementia.
In patients with severe renal impairment (CrCl < 10 mL/min), the dosing interval should be increased (e.g., every 12 to 18 hours) to prevent the accumulation of metabolites, although the parent drug itself is mostly metabolized by the liver.
Patients with cirrhosis or other forms of liver disease may have a reduced ability to clear the drug from their system. This can lead to a longer half-life and increased risk of side effects. Dose reductions are often required for these patients.
> Important: Special populations require individualized medical assessment.
Diphenhydramine is a competitive antagonist of the Histamine-1 (H1) receptor. Histamine normally binds to H1 receptors to trigger inflammatory and allergic responses. By occupying these receptor sites, diphenhydramine prevents histamine from binding, thereby inhibiting the allergic cascade. Additionally, it acts as a muscarinic antagonist, blocking acetylcholine's action at parasympathetic nerve endings, which results in its 'drying' (anticholinergic) effects. Its sedative properties stem from its ability to cross the blood-brain barrier and block H1 receptors in the tuberomammillary nucleus of the hypothalamus, which is involved in maintaining wakefulness.
The onset of action for oral diphenhydramine is typically 15 to 30 minutes, with the maximum therapeutic effect occurring between 1 and 3 hours. The duration of action generally lasts between 4 and 8 hours. While it is effective for acute symptom relief, chronic use can lead to pharmacological tolerance, particularly regarding its sedative effects.
| Parameter | Value |
|---|---|
| Bioavailability | 40% - 60% |
| Protein Binding | 78% - 99% |
| Half-life | 2.4 - 9.3 hours |
| Tmax | 1 - 4 hours |
| Metabolism | Hepatic (Primary: CYP2D6) |
| Excretion | Renal (94%) |
Diphenhydramine is a first-generation antihistamine within the ethanolamine class. Related medications in this class include clemastine and dimenhydrinate. Unlike second-generation antihistamines (e.g., fexofenadine), it is characterized by high lipid solubility and significant CNS penetration.
Common questions about Allergy Relief Caplet
Diphenhydramine is primarily used to relieve symptoms of hay fever, upper respiratory allergies, and the common cold, such as sneezing, runny nose, and itchy eyes. It is also frequently utilized as a short-term sleep aid for occasional insomnia due to its potent sedative properties. In some clinical settings, healthcare providers may use it to treat motion sickness or manage early symptoms of Parkinson’s disease, including tremors. Additionally, it is an effective treatment for allergic skin reactions like hives or insect bites. Always consult with a healthcare professional to ensure the medication is appropriate for your specific symptoms.
The most common side effect of diphenhydramine is significant drowsiness, which can impair your ability to perform tasks requiring mental alertness. Many patients also experience anticholinergic effects, such as a very dry mouth, nose, and throat. Other frequent issues include dizziness, blurred vision, and a feeling of 'heaviness' in the limbs. In some cases, gastrointestinal upset like constipation or nausea may occur. These effects are generally dose-dependent and subside as the drug is cleared from the body.
No, you should strictly avoid alcohol while taking diphenhydramine. Both substances are central nervous system depressants, and combining them can dangerously intensify sedation and respiratory depression. This combination significantly increases the risk of accidents, severe dizziness, and loss of consciousness. Even small amounts of alcohol can react poorly with the medication. Always wait until the drug is completely out of your system before consuming alcoholic beverages.
Diphenhydramine is often used during pregnancy for allergies or nausea, but it should only be taken under the explicit guidance of a healthcare provider. While it is generally considered to have a low risk of causing birth defects, some studies suggest caution, particularly in the first and third trimesters. High doses near delivery may potentially cause uterine irritability or withdrawal symptoms in the newborn. Your doctor will weigh the benefits against the potential risks for your specific situation. Always inform your obstetrician before starting any over-the-counter medication.
When taken orally, diphenhydramine typically begins to work within 15 to 30 minutes. You will likely feel the peak effects, including maximum sedation or allergy relief, within 1 to 3 hours after ingestion. The effects generally last for about 4 to 8 hours in most healthy adults. If you are using it as a sleep aid, it is best taken about 30 minutes before your intended bedtime. Factors like age, liver function, and whether you have eaten can slightly alter these timelines.
For most people using diphenhydramine occasionally for allergies, stopping the medication suddenly does not cause physical withdrawal. However, if you have been using it every night as a sleep aid, you might experience 'rebound insomnia,' where it becomes even harder to fall asleep for a few nights. It is generally better to use the medication only as needed rather than every day. If you find you cannot sleep without it, consult your doctor to address the underlying cause of your insomnia. They can help you transition to a more sustainable sleep management plan.
If you miss a dose of diphenhydramine, take it as soon as you remember, provided it is not almost time for your next dose. If your next scheduled dose is only a few hours away, skip the missed dose entirely and continue with your regular timing. Never take two doses at once to make up for a missed one, as this significantly increases the risk of toxic side effects. Since it is often taken on an 'as-needed' basis, missing a dose is usually not a clinical emergency. Simply return to your prescribed or recommended schedule for the next dose.
Weight gain is not a commonly reported side effect of short-term diphenhydramine use. However, some research into long-term use of first-generation antihistamines suggests they may interfere with the body's satiety signals, potentially leading to increased appetite. Additionally, the sedation caused by the drug might lead to decreased physical activity in some individuals. If you notice significant weight changes while taking this medication, discuss them with your healthcare provider. They can help determine if the drug or another underlying factor is responsible.
Diphenhydramine can interact with many other drugs, particularly those that also cause drowsiness or have anticholinergic properties. It should not be combined with other antihistamines, opioid pain relievers, muscle relaxants, or anxiety medications without medical supervision. It also interacts with certain antidepressants and blood pressure medications by inhibiting the CYP2D6 enzyme. Always provide your doctor or pharmacist with a full list of your current medications to check for safety. This includes herbal supplements and other over-the-counter products.
Yes, diphenhydramine is widely available as a generic medication and is the active ingredient in many brand-name products like Benadryl, ZzzQuil, and Aleve PM. Generic versions are required by the FDA to have the same quality, strength, and purity as brand-name versions. Choosing a generic can often be a more cost-effective way to treat your symptoms. You can find generic diphenhydramine in various forms, including tablets, capsules, and liquids, at most pharmacies and grocery stores. Always check the label to ensure diphenhydramine hydrochloride is the only active ingredient if you are seeking a single-ingredient product.
Other drugs with the same active ingredient (Diphenhydramine)