Diphenhydramine 25mg Capsules
It is used to ease allergy signs.
Drug Class
Antihistamine, Sedative, Antitussive, Antiemetic
Pharmacologic Class
First-generation H1-receptor antagonist, Anticholinergic
Pregnancy Category
B
FDA Approved
Mar 1946
DEA Schedule
Not Controlled
Overview
What is this medicine?
Diphenhydramine is an antihistamine used to relieve allergy symptoms like sneezing and runny nose. It can also cause drowsiness, making it useful as a sleep aid for occasional insomnia. It works by blocking a natural substance (histamine) that your body makes during an allergic reaction.
How to Use This Medicine
Taking Your Medication
To use this medication correctly, follow your doctor's instructions and read all the information provided. You can take this medication with or without food. If it causes stomach upset, taking it with food may help.
Storing and Disposing of Your Medication
Keep your medication at room temperature, away from light and moisture. Avoid storing it in a bathroom. Ensure all medications are kept in a safe location, out of reach of children and pets. When disposing of unused or expired medication, do not flush it down the toilet or pour it down the drain unless instructed to do so. Instead, consult your pharmacist for guidance on proper disposal. You may also want to inquire about drug take-back programs available in your area.
Missing a Dose
If you take this medication regularly and miss a dose, take it as soon as you remember. However, if it's close to the time for your next dose, skip the missed dose and resume your regular schedule. Do not take two doses at the same time or take extra doses. If you take this medication as needed, follow your doctor's instructions and do not take it more frequently than recommended.
To use this medication correctly, follow your doctor's instructions and read all the information provided. You can take this medication with or without food. If it causes stomach upset, taking it with food may help.
Storing and Disposing of Your Medication
Keep your medication at room temperature, away from light and moisture. Avoid storing it in a bathroom. Ensure all medications are kept in a safe location, out of reach of children and pets. When disposing of unused or expired medication, do not flush it down the toilet or pour it down the drain unless instructed to do so. Instead, consult your pharmacist for guidance on proper disposal. You may also want to inquire about drug take-back programs available in your area.
Missing a Dose
If you take this medication regularly and miss a dose, take it as soon as you remember. However, if it's close to the time for your next dose, skip the missed dose and resume your regular schedule. Do not take two doses at the same time or take extra doses. If you take this medication as needed, follow your doctor's instructions and do not take it more frequently than recommended.
Lifestyle & Tips
- Avoid driving or operating heavy machinery until you know how this medication affects you, as it can cause significant drowsiness.
- Avoid alcohol and other medications that cause drowsiness (e.g., sedatives, tranquilizers, pain medications) while taking diphenhydramine, as this can increase sedation and other side effects.
- Do not exceed the recommended dose.
- For insomnia, take 30 minutes before bedtime.
Available Forms & Alternatives
Available Strengths:
- Diphenhydramine 50mg/ml Inj, 1ml
- Diphenhydramine Hcl 50mg Caps
- Diphenhydramine 50mg Capsules
- Diphenhydramine 12.5mg/5ml Elixir
- Diphenhydramine 25mg Tablets
- Diphenhydramine 25mg/10ml Liquid
- Diphenhydramine 12.5mg/5ml Liquid
- Diphenhydramine 12.5mg/5ml Liquid
- Diphenhydramine 25mg Caps
- Diphenhydramine 25mg Capsules
- Diphenhydramine 50mg/ml Inj, 1ml
- Diphenhydramine 25mg Tablets
- Diphenhydramine 25mg Capsules
Dosing & Administration
Adult Dosing
Standard Dose:
25-50 mg every 4-6 hours as needed (for allergy/cold symptoms); 50 mg at bedtime (for insomnia)
Dose Range:
25 - 50 mg
Condition-Specific Dosing:
allergy_cold:
Max 300 mg/day
insomnia:
Max 50 mg/day
Pediatric Dosing
Neonatal:
Not established
Infant:
Not established (generally not recommended under 2 years)
Child:
6-12 years: 12.5-25 mg every 4-6 hours as needed (Max 150 mg/day)
Adolescent:
12 years and older: 25-50 mg every 4-6 hours as needed (Max 300 mg/day)
Dose Adjustments
Renal Impairment:
Mild:
No specific adjustment, but monitor for increased sedation
Moderate:
Consider lower doses (e.g., 25 mg) and extended dosing intervals (e.g., every 6-8 hours)
Severe:
Consider lower doses (e.g., 25 mg) and extended dosing intervals (e.g., every 8-12 hours)
Dialysis:
Not significantly removed by dialysis; adjust based on clinical response and side effects
Hepatic Impairment:
Mild:
No specific adjustment, but monitor for increased sedation
Moderate:
Consider lower doses (e.g., 25 mg) and extended dosing intervals
Severe:
Consider lower doses (e.g., 25 mg) and extended dosing intervals
Confidence:
Medium
Pharmacology
Mechanism of Action
Diphenhydramine is a first-generation antihistamine that competitively blocks H1 histamine receptors in the periphery and central nervous system (CNS). Its sedative effects are due to its ability to readily cross the blood-brain barrier and block H1 receptors in the CNS. It also possesses significant anticholinergic (muscarinic), antiemetic, and local anesthetic properties.
Pharmacokinetics
Absorption:
Bioavailability:
40-60% (due to first-pass metabolism)
Tmax:
1-4 hours
FoodEffect:
Food may delay absorption but does not significantly affect the extent of absorption.
Distribution:
Vd:
17 L/kg
ProteinBinding:
78-85%
CnssPenetration:
Yes
Elimination:
HalfLife:
2.4-9.3 hours (average 4-8 hours)
Clearance:
Not readily available (highly variable)
ExcretionRoute:
Primarily renal (as metabolites)
Unchanged:
<1%
Pharmacodynamics
OnsetOfAction:
15-60 minutes
PeakEffect:
1-4 hours
DurationOfAction:
4-6 hours (antihistamine/sedative effects)
Safety & Warnings
Side Effects
Urgent Side Effects: Seek Medical Help Right Away
If you experience any of the following symptoms, call your doctor immediately or seek emergency medical attention, as they may be signs of a severe and potentially life-threatening reaction:
Signs of an allergic reaction, such as:
+ Rash
+ Hives
+ Itching
+ Red, swollen, blistered, or peeling skin with or without fever
+ Wheezing
+ Tightness in the chest or throat
+ Trouble breathing, swallowing, or talking
+ Unusual hoarseness
+ Swelling of the mouth, face, lips, tongue, or throat
Severe dizziness or fainting
Change in balance
Feeling less alert
Other Possible Side Effects
While many people may not experience any side effects or only minor ones, it's essential to be aware of the following potential side effects. If you experience any of these or other side effects that bother you or do not go away, contact your doctor for advice:
Dizziness or drowsiness
Thickening of mucus in the nose or throat
Feeling nervous and excitable
Upset stomach or vomiting
Reporting Side Effects
This is not an exhaustive list of possible side effects. If you have questions or concerns about side effects, consult your doctor. You can also report side effects to the FDA at 1-800-332-1088 or online at https://www.fda.gov/medwatch.
If you experience any of the following symptoms, call your doctor immediately or seek emergency medical attention, as they may be signs of a severe and potentially life-threatening reaction:
Signs of an allergic reaction, such as:
+ Rash
+ Hives
+ Itching
+ Red, swollen, blistered, or peeling skin with or without fever
+ Wheezing
+ Tightness in the chest or throat
+ Trouble breathing, swallowing, or talking
+ Unusual hoarseness
+ Swelling of the mouth, face, lips, tongue, or throat
Severe dizziness or fainting
Change in balance
Feeling less alert
Other Possible Side Effects
While many people may not experience any side effects or only minor ones, it's essential to be aware of the following potential side effects. If you experience any of these or other side effects that bother you or do not go away, contact your doctor for advice:
Dizziness or drowsiness
Thickening of mucus in the nose or throat
Feeling nervous and excitable
Upset stomach or vomiting
Reporting Side Effects
This is not an exhaustive list of possible side effects. If you have questions or concerns about side effects, consult your doctor. You can also report side effects to the FDA at 1-800-332-1088 or online at https://www.fda.gov/medwatch.
Seek Immediate Medical Attention If You Experience:
- Severe drowsiness or dizziness
- Confusion or hallucinations
- Difficulty urinating
- Severe dry mouth or blurred vision
- Fast or irregular heartbeat
- Seizures (in overdose)
Before Using This Medicine
Before Taking This Medication: Important Information to Share with Your Doctor
It is essential to inform your doctor about the following:
Any allergies you have, including allergies to this medication, its components, or other substances. Be sure to describe the symptoms you experienced.
If you are breastfeeding or plan to breastfeed, as you may need to avoid breastfeeding while taking this medication.
If the patient is a premature baby or newborn, as this form of the medication should not be given to them.
This is not an exhaustive list of all potential interactions with this medication. Therefore, it is crucial to discuss the following with your doctor and pharmacist:
All medications you are currently taking, including prescription and over-the-counter drugs, natural products, and vitamins.
* Any health problems you have, as they may interact with this medication.
To ensure your safety, always check with your doctor before starting, stopping, or changing the dose of any medication.
It is essential to inform your doctor about the following:
Any allergies you have, including allergies to this medication, its components, or other substances. Be sure to describe the symptoms you experienced.
If you are breastfeeding or plan to breastfeed, as you may need to avoid breastfeeding while taking this medication.
If the patient is a premature baby or newborn, as this form of the medication should not be given to them.
This is not an exhaustive list of all potential interactions with this medication. Therefore, it is crucial to discuss the following with your doctor and pharmacist:
All medications you are currently taking, including prescription and over-the-counter drugs, natural products, and vitamins.
* Any health problems you have, as they may interact with this medication.
To ensure your safety, always check with your doctor before starting, stopping, or changing the dose of any medication.
Precautions & Cautions
It is essential to inform all your healthcare providers, including doctors, nurses, pharmacists, and dentists, that you are taking this medication.
To minimize the risk of severe side effects, do not exceed the dosage prescribed by your doctor. Taking more than the recommended amount can increase your chance of experiencing adverse effects. Additionally, do not take this medication for a longer duration than specified by your doctor.
Until you are familiar with how this medication affects you, avoid operating a vehicle or engaging in activities that require alertness.
Do not use this medication in conjunction with other products containing diphenhydramine. It is also recommended to avoid consuming alcohol while taking this drug.
Before using marijuana, cannabis, or prescription/over-the-counter medications that may cause drowsiness, consult with your doctor.
Note that different brands of this medication may be suitable for children of varying ages. Prior to administering this medication to a child, consult with their doctor. Furthermore, do not use this medication as a means to induce sleep in children; instead, discuss alternative solutions with their doctor.
If you are 60 years or older, exercise caution when using this medication, as you may be more susceptible to side effects. Similarly, when administering this medication to children, use it with caution, as they may have a higher risk of experiencing excitability.
If you are pregnant or planning to become pregnant, inform your doctor, as they will need to discuss the potential benefits and risks associated with using this medication during pregnancy.
To minimize the risk of severe side effects, do not exceed the dosage prescribed by your doctor. Taking more than the recommended amount can increase your chance of experiencing adverse effects. Additionally, do not take this medication for a longer duration than specified by your doctor.
Until you are familiar with how this medication affects you, avoid operating a vehicle or engaging in activities that require alertness.
Do not use this medication in conjunction with other products containing diphenhydramine. It is also recommended to avoid consuming alcohol while taking this drug.
Before using marijuana, cannabis, or prescription/over-the-counter medications that may cause drowsiness, consult with your doctor.
Note that different brands of this medication may be suitable for children of varying ages. Prior to administering this medication to a child, consult with their doctor. Furthermore, do not use this medication as a means to induce sleep in children; instead, discuss alternative solutions with their doctor.
If you are 60 years or older, exercise caution when using this medication, as you may be more susceptible to side effects. Similarly, when administering this medication to children, use it with caution, as they may have a higher risk of experiencing excitability.
If you are pregnant or planning to become pregnant, inform your doctor, as they will need to discuss the potential benefits and risks associated with using this medication during pregnancy.
Overdose Information
Overdose Symptoms:
- Severe drowsiness
- Confusion
- Hallucinations
- Dilated pupils
- Flushed skin
- Fever
- Dry mouth
- Urinary retention
- Seizures
- Coma
- Respiratory depression
What to Do:
Seek immediate medical attention. Call 911 or Poison Control at 1-800-222-1222. Treatment is supportive and symptomatic, potentially including activated charcoal if recent ingestion, and rarely physostigmine for severe anticholinergic toxicity.
Drug Interactions
Contraindicated Interactions
- Monoamine Oxidase Inhibitors (MAOIs) concurrently or within 14 days (can prolong and intensify anticholinergic and CNS depressant effects)
Major Interactions
- CNS depressants (e.g., alcohol, opioids, benzodiazepines, hypnotics, sedatives, anxiolytics, tricyclic antidepressants, antipsychotics) - increased sedation and respiratory depression
- Other anticholinergic drugs (e.g., atropine, scopolamine, tricyclic antidepressants, some antipsychotics) - increased risk of anticholinergic side effects (dry mouth, blurred vision, urinary retention, constipation)
Moderate Interactions
- Potassium chloride (oral solid dosage forms) - increased risk of GI lesions due to delayed gastric emptying
- Betahistine - theoretical antagonism of effects
Minor Interactions
- None specifically categorized as minor, but general caution with any drug that can cause drowsiness or anticholinergic effects.
Monitoring
Symptom Monitoring
- Drowsiness
- Dizziness
- Dry mouth
- Blurred vision
- Urinary retention
- Constipation
- Paradoxical excitation (especially in children)
- Confusion (especially in elderly)
Special Patient Groups
Pregnancy
Generally considered safe for occasional use during pregnancy (Category B), but use with caution and only if clearly needed. Avoid in late third trimester due to potential for neonatal withdrawal symptoms (irritability, tremors) or increased risk of retrolental fibroplasia in premature infants.
Trimester-Specific Risks:
First Trimester:
No clear evidence of increased risk of major birth defects.
Second Trimester:
Generally considered low risk for occasional use.
Third Trimester:
Avoid in late third trimester due to potential for neonatal withdrawal symptoms or increased risk of retrolental fibroplasia in premature infants.
Lactation
Excreted into breast milk. Use with caution. Can cause drowsiness, irritability, or paradoxical excitation in breastfed infants. May also decrease milk supply due to anticholinergic effects. Consider alternatives, especially in newborns or premature infants.
Infant Risk:
L3 (Moderate risk)
Pediatric Use
Use with caution, especially in children under 6 years old. Not recommended for infants. Children may experience paradoxical excitation (hyperactivity, agitation) instead of sedation. Always use pediatric-specific dosing and formulations when available. Avoid use as a sedative in children under 2 years due to risk of respiratory depression.
Geriatric Use
Increased sensitivity to anticholinergic effects (drowsiness, confusion, urinary retention, constipation, dry mouth, blurred vision, increased risk of falls). Use lower doses and with extreme caution. Avoid in elderly patients with dementia or cognitive impairment due to increased risk of cognitive decline. Listed on the Beers Criteria as a medication to avoid in older adults due to high anticholinergic burden.
Clinical Information
Clinical Pearls
- Diphenhydramine is a common cause of anticholinergic toxicity, especially in overdose or in susceptible populations (elderly, children).
- Tolerance to the sedative effects can develop with continued use, making it less effective as a sleep aid over time.
- Despite its OTC status, it has significant side effects and drug interactions, particularly with other CNS depressants and anticholinergic agents.
- Often used off-label for nausea/vomiting (due to antiemetic properties) or motion sickness.
- Patients should be advised about the potential for next-day sedation if used for sleep.
Alternative Therapies
- For allergies: Second-generation antihistamines (e.g., loratadine, cetirizine, fexofenadine) which are less sedating and have fewer anticholinergic effects.
- For insomnia: Melatonin, doxylamine (another first-generation antihistamine), non-benzodiazepine hypnotics (e.g., zolpidem, eszopiclone - prescription), cognitive behavioral therapy for insomnia (CBT-I).
- For cough: Dextromethorphan, guaifenesin.
- For motion sickness: Scopolamine patch, meclizine.
Cost & Coverage
Generic Available:
Yes
Insurance Coverage:
Over-the-counter (OTC), generally not covered by prescription insurance unless prescribed by a physician.
General Drug Facts
If your symptoms or health issues persist or worsen, it's essential to contact your doctor for further guidance. To ensure your safety, never share your medication with others or take someone else's medication. Some medications may come with an additional patient information leaflet, so be sure to check with your pharmacist for more information. If you have any questions or concerns about this medication, don't hesitate to reach out to your doctor, nurse, pharmacist, or other healthcare provider. In the event of a suspected overdose, immediately call your local poison control center or seek medical attention. When seeking help, be prepared to provide details about the overdose, including the medication taken, the amount, and the time it occurred.