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What is Diphenhydramine?

Hillary Flynn
By
Updated Feb 05, 2024
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Diphenhydramine, most commonly known by the trade name Benadryl®, is an antihistamine. Antihistamines combat the effects of histamine released during allergic reactions, which can cause hives, itching, sneezing, and congestion. Diphenhydramine is also used to treat motion sickness, nausea, coughing, and insomnia. Less commonly, it is used to treat the muscle spasms associated with mild forms of Parkinson's disease as well as the abnormal muscle movements caused by psychiatric drugs in the phenothiazine family.

Diphenhydramine is an over-the-counter drug when used in tablet, capsule, or liquid form, but is also prescribed as an injection. Injections are typically only used in medical settings to treat allergic reactions to blood or in lieu of epinephrine. It works as an antihistamine by blocking cell receptors before histamine can bind with a cell. When the antihistamine binds instead, the cell is not stimulated, and hence, the allergic symptoms are not produced. This also succeeds in blocking acetylcholine action, which is not necessarily a desired effect for those taking diphenhydramine for its antihistamine benefits.

Blocking acetylcholine action is called the anticholinergic effect. This can cause constipation, dry mouth, blurry vision, dizziness, and drowsiness. The drowsiness associated with diphenhydramine is, however, the sought out effect of the medication when it is purchased as a sleep aid in over-the-counter drugs such as Unisom® or Nytol®. When used as a sleep aid, it is not recommended for extended use. If sleep issues persist for longer than two weeks, a doctor should be consulted as a tolerance to the sedating effects of diphenhydramine develop quickly.

The full range of side effects associated with diphenhydramine include: drowsiness, dizziness, constipation, blurred vision, coordination issues, drying of respiratory secretions, low blood pressure, heart palpitations, motor impairment, sensitivity to light, difficulty urinating, short term memory loss, difficulty concentrating, decreased appetite, upset stomach, hallucinations, and irritability.

Anyone taking other sedating medications such as anti-anxiety medications, narcotic pain relievers, anti-depressants, sleeping pills, or alcohol should use caution as the drowsiness associated with diphenhydramine will be markedly increased. Anyone over sixty may also notice a stronger sedative effect, so dosages may need to be decreased. No one taking this medication should operate heavy machinery or a motor vehicle until the sedative effects have worn off.

Diphenhydramine is secreted in breast milk, so nursing mothers should not take it. Those with the following medical conditions should consult a doctor before using this medication: asthma, glaucoma, enlarged prostate gland, ulcers, heart disease, high blood pressure, seizures, thyroid problems, or bowel obstructions.

WiseGeek is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Hillary Flynn
By Hillary Flynn
Hillary Flynn's insatiable curiosity led her to join the WiseGeek team, where she contributes well-researched articles on various topics. In addition to her work with WiseGeek, Hillary manages an electronic publishing business that allows her to develop her skills in technical writing, graphic design, and business development. With a passion for satirical writing and traveling to historical places, Hillary brings a distinctive voice to her content.
Discussion Comments
By anon311767 — On Jan 03, 2013

I need help. I am very dependent on benadryl for sleeping. I don't want to take it anymore. I am afraid of just stopping because I don't know what side effects would result from that. Someone please help me.

By pastanaga — On Oct 03, 2011

@Iluviaporos - Taking sleeping pills a couple of times isn't going to hurt though. Even if they can tend to reduce your REM sleep, they will probably not do that all night. So, they can help people get over the initial difficulty in getting to sleep, and then once they wear off, the person will be able to get the benefits of sleep.

I didn't know the same active ingredient in Benadryl was used in sleeping pills as well. I should really pay more attention to what is written on pill boxes at the pharmacy.

By lluviaporos — On Oct 03, 2011

Well, I've used Bendryl several times and never had any ill effects. It does make you a bit sleepy, but it doesn't exactly knock you out. Especially if you are taking the recommended dose.

But I've only ever used it for allergies. I'm a bit against using sleeping pills unless it's absolutely necessary, because I've heard they can repress REM sleep, which is the main reason you need sleep in the first place. So, even though they might make you feel like you are sleeping well, in reality you will get up feeling more tired than when you went to bed.

Of course, people shouldn't be taking Bendryl in order to sleep anyway, and it can be dangerous if they take too much.

If you are interested in that effect of diphenhydramine, you should try the medications which include it specifically as a sleeping aid, so you get the right dose and instructions.

By Mor — On Oct 02, 2011

I've always been a bit nervous about taking Benadryl, because a friend of mine told me that he thought his mother was addicted to the stuff.

It didn't really hurt her, in his opinion, but she felt like she needed her "bennies" in order to get to sleep at night.

A couple of times when I've had insomnia, I've been tempted to try taking one for that reason, but I would hate to be addicted to something. And from the sound of it, the effects would quickly wear off, making it even more difficult to get to sleep.

I guess if it really bothered me I should get a proper prescription for sleeping pills.

Hillary Flynn
Hillary Flynn
Hillary Flynn's insatiable curiosity led her to join the WiseGeek team, where she contributes well-researched articles...
Learn more
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