Diphenhydramine
Over-the-counterWhat is Diphenhydramine?
Diphenhydramine is an antihistamine medication available without a prescription. It works by blocking histamine, a natural substance your body makes during allergic reactions. Many people know this medication by brand names like Benadryl or ZzzQuil.
This medication treats allergy symptoms like sneezing, runny nose, and itchy eyes. It also causes drowsiness, which makes it useful for occasional sleeplessness. You can find diphenhydramine in tablets, capsules, liquids, and topical creams.
Diphenhydramine belongs to a class of drugs called first-generation antihistamines. These medications have been used safely for over 70 years. While effective, they cause more drowsiness than newer antihistamine options.
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Common uses
- Seasonal allergies and hay fever symptoms
- Allergic reactions to food, insect bites, or environmental triggers
- Itching from hives, eczema, or other skin conditions
- Runny nose and sneezing from the common cold
- Motion sickness and nausea prevention
- Occasional sleeplessness or difficulty falling asleep
- Mild cough due to throat irritation
How it works
Diphenhydramine blocks histamine receptors throughout your body. Histamine is a chemical that causes allergy symptoms like itching, sneezing, and watery eyes. When you take diphenhydramine, it prevents histamine from attaching to cells and triggering these reactions.
This medication also crosses into your brain, where it causes drowsiness. It blocks histamine and acetylcholine receptors in the central nervous system. This is why diphenhydramine makes you sleepy and is sold as a sleep aid. The drowsy effect typically starts within 30 to 60 minutes and lasts 4 to 6 hours.
Taking Diphenhydramine? Monitor your levels.
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Dosage information
Adults and children 12 years and older typically take 25 to 50 mg every 4 to 6 hours. Do not take more than 300 mg in 24 hours. Children aged 6 to 11 may take 12.5 to 25 mg every 4 to 6 hours, not exceeding 150 mg daily. For sleep, adults usually take 50 mg at bedtime. Always follow the package directions or ask your pharmacist for guidance on the right dose for your needs.
Side effects
- Drowsiness and tiredness
- Dry mouth, nose, and throat
- Dizziness or lack of coordination
- Upset stomach or nausea
- Blurred vision
- Constipation or difficulty urinating
- Thickening of mucus in airways
Serious side effects are rare but require immediate medical attention. Stop taking diphenhydramine and seek help if you experience fast or irregular heartbeat, severe dizziness, confusion, difficulty urinating, or seizures. Some people, especially children and older adults, may experience excitement or restlessness instead of drowsiness.
Drug interactions
- Alcohol increases drowsiness and dizziness risk
- Other antihistamines or cold medications may cause overdose
- Sleep medications, anxiety drugs, or muscle relaxants increase sedation
- MAO inhibitors taken within 14 days can cause serious reactions
- Anticholinergic drugs like certain antidepressants increase side effects
- Avoid use with other products containing diphenhydramine, including topical creams
Blood test monitoring
Diphenhydramine does not typically require blood test monitoring for short-term use. However, if you use this medication regularly or have certain health conditions, your doctor may want to check your liver and kidney function. These organs process and remove diphenhydramine from your body.
People with glaucoma, enlarged prostate, or urinary problems should have regular checkups. Your doctor may monitor your eye pressure or urinary function. If you take diphenhydramine for extended periods, Rite Aid offers comprehensive blood testing with over 200 biomarkers. Regular testing helps ensure your liver and kidneys stay healthy.
Related blood tests
Taking Diphenhydramine? Monitor your levels.
- Track how your body responds
- Catch side effects early with blood work
- Share results with your doctor
Frequently asked questions
Yes, diphenhydramine is available over the counter at Rite Aid and other pharmacies. You do not need a prescription to purchase it. However, ask your pharmacist if you have questions about whether it is right for you or how to use it safely.
Diphenhydramine typically stays in your body for 4 to 6 hours. However, it can be detected in your system for up to 24 hours after your last dose. The drowsy effects usually wear off after 4 to 6 hours, but some people feel groggy the next morning.
Diphenhydramine is meant for occasional sleeplessness, not daily use. Your body can build tolerance, making it less effective over time. Taking it every night may also cause side effects like dry mouth, confusion, and constipation. Talk to your doctor if you have ongoing sleep problems.
No, you should not drive or operate machinery after taking diphenhydramine. This medication causes drowsiness and slows your reaction time. These effects can last for several hours. Wait until the medication has worn off completely and you feel fully alert before driving.
The medication is exactly the same. Products marketed for sleep often contain 50 mg, while allergy products may contain 25 mg per dose. The only difference is the packaging and marketing. Both versions work the same way in your body.
Older adults should use diphenhydramine with caution. They are more sensitive to side effects like confusion, dizziness, and difficulty urinating. The American Geriatrics Society recommends avoiding diphenhydramine in people over 65 when possible. Ask your doctor about safer alternatives for allergies or sleep.
Diphenhydramine can sometimes cause false positive results on drug tests. It may show up as methadone or PCP on certain screening tests. If you take diphenhydramine regularly, tell the testing facility beforehand. A confirmatory test can distinguish diphenhydramine from other substances.
Diphenhydramine is generally considered safe during pregnancy and breastfeeding. However, you should talk to your doctor before using any medication while pregnant. Small amounts pass into breast milk and may cause drowsiness in nursing infants.
Taking too much diphenhydramine can be dangerous. Overdose symptoms include extreme drowsiness, confusion, fast heartbeat, flushed skin, dilated pupils, or seizures. Call poison control at 1-800-222-1222 or go to the emergency room immediately if you take more than the recommended dose.
Yes, Rite Aid offers comprehensive blood testing for $349 per year. You get 2 tests annually measuring over 200 biomarkers including liver and kidney function. While diphenhydramine does not usually require monitoring, regular testing helps track your overall health. This is especially helpful if you use diphenhydramine frequently or have other health conditions.