Does Benadryl sometimes not work
How Does Benadryl Work In The Body?
Nov 05, 2018
Ellen asked
I am hoping you can answer exactly how Benadryl works. What is the mechanism of action? I have heard and read conflicting things.
At a glance
- Benadryl is a sedating antihistamine that works bycompeting with histamine for binding at H1-receptor sites.
Answer
Benadryl (diphenhydramine) is an antihistamine, which are divided into several classes, each with unique properties. Benadryl is in the ethanolamine class of antihistamines.
Antihistamines in the ethanolamine class (which also includes doxylamine, the active ingredient in NyQuil) are most well known for their anticholinergic (specifically anti-muscarinic) activity. This causes pronounced sedation in most individuals.
Additionally, due to its anticholinergic properties, Benadryl is effective in the relief of nausea, vomiting, and vertigo associated with motion sickness.
Newer antihistamines, such as Claritin and Allegra, don't have significant anticholinergic properties and don't tend to cause sedation.
Benadryl Mechanism Of Action
All antihistamines work in the same manner, by competing with free histamine for binding at H1-receptor sites. For this reason, antihistamines are also known as H1-antagonists. Antihistamines do not prevent the release of histamine.
As a side note, there are antihistamines that are H2-antagonists, such as Zantac and Pepcid. These work by blocking H2-receptors, mostly in the stomach and GI tract, which reduces acid secretion. For this article, we will use 'antihistamine' to refer to an H1-antagonist.
Benadryl competitively antagonizes the effects of histamine on H1-receptors in a variety of places in the body, including:
- GI tract
- Uterus
- Blood vessels
- Bronchial muscle
Preventing the binding of histamine to its respective receptors in the body makes it useful for suppressing inflammation, decreasing itching and reducing swelling, all of which are common symptoms of allergies.
As mentioned above, antihistamines have anticholinergic properties, but to varying degrees. This is a key differentiating factor as to why different antihistamines have different effects on individuals.
Benadryl, being an ethanolamine antihistamine, has far greater anticholinergic activity than do other antihistamines. This is why, in addition to allergy relief, Benadryl is useful for:
- Treatment of nausea/vomiting
- Treatment of motion sickness
- Treatment of insomnia (Benadryl is commonly used as a sleep aid)
While the anticholinergic activity of Benadryl can be beneficial, it is also responsible for many of its potential side effects, including:
- Dry eyes
- Dry mouth
- Constipation
- CNS depression
- Dizziness
- Movement disorders (rare)
- Cardiac abnormalities (rare)
Benadryl is also available in topical dosage forms, which provides local relief from:
- Insect bites
- Minor burns
- Minor abrasions
The topical effects are likely due to the antihistamine activity but also possibly due to a mild anesthetic effect resulting from the decreased transmission of nerve impulses.
References
- ClinicalKey: Diphenhydramine Monograph. ClinicalKey (Subscription Required)
- Diphenhydramine as an Alternative Local Anesthetic Agent. PubMed
- Efficacy of diphenhydramine vs desloratadine and placebo in patients with moderate-to-severe seasonal allergic rhinitis. PubMed
- Benadryl Prescribing Information. AccessFDA
Benadryl (diphenhydramine)
You may wonder how Benadryl compares to other medications approved for similar uses. Below are comparisons between Benadryl and several medications.
Benadryl vs. Claritin
Benadryl is a first-generation antihistamine. Claritin (loratadine) is a newer, second-generation antihistamine. Second-generation antihistamines are often called nonsedating antihistamines because theyre less likely to cause sleepiness than first-generation antihistamines.
Both Benadryl and Claritin are over-the-counter (OTC) medications.
Uses
Oral Benadryl products are approved for decreasing symptoms of hay fever and other respiratory allergies, and symptoms of the common cold, such as sneezing and runny nose. Topical Benadryl products are approved for decreasing pain and itchy skin resulting from hives, insect bites, and other causes.
Claritin is approved for decreasing symptoms of hay fever and other respiratory allergies.
Drug forms
Benadryl comes in many different forms, including:
- oral tablets
- oral liquid-filled capsules (liqui-gels)
- oral chewable tablets
- oral liquid solution
- topical cream
- topical gel
- topical spray
- topical stick
Oral Benadryl products are usually taken every 4 to 6 hours. Topical products are typically used up to four times daily.
Claritin is also available in many different forms, including:
- oral tablets
- orally disintegrating tablets
- oral liquid-filled capsules (liqui-gels)
- oral liquid syrup
Claritin tablets, liquid-filled capsules, and syrup are taken once daily. The orally disintegrating tablets are used either once daily or twice daily.
Side effects and risks
Benadryl and Claritin have some similar side effects, and some that differ. Below are examples of these side effects.
* Both Benadryl and Claritin can cause sleepiness, but its much more common in people who take Benadryl.
Effectiveness
Benadryl and Claritin are both effective for reducing symptoms of hay fever and other allergies, and for treating hives or itchy skin. However, Benadryl isnt usually a first-choice treatment for these conditions due to its risk of side effects such as sleepiness.
Claritin and other second-generation antihistamines are usually preferred.
Costs
Benadryl and Claritin are both brand-name OTC products. Claritin usually costs more than Benadryl.
Both of these products have store-brand versions. Store brands are usually cheaper than the brand-name versions.
Benadryl vs. Zyrtec
Benadryl is a first-generation antihistamine. Zyrtec (cetirizine) is a newer, second-generation antihistamine. Second-generation antihistamines are often called nonsedating antihistamines because theyre less likely to cause sleepiness than first-generation antihistamines.
Both Benadryl and Zyrtec are OTC medications.
Uses
Oral Benadryl products are approved for decreasing symptoms of hay fever and other respiratory allergies, and symptoms of the common cold, such as sneezing and runny nose. Topical Benadryl products are approved for decreasing pain and itchy skin due to hives, insect bites, and other causes.
Zyrtec is approved for decreasing symptoms of hay fever and other respiratory allergies.
Drug forms
Benadryl is available in many different forms, including:
- oral tablets
- oral liquid-filled capsules (liqui-gels)
- oral chewable tablets
- oral liquid solution
- topical cream
- topical gel
- topical spray
- topical stick
Oral Benadryl products are usually taken every 4 to 6 hours. Topical Benadryl products are typically used up to four times daily.
Zyrtec is also available in many different forms, including:
- oral tablets
- orally disintegrating tablets (dissolve tabs)
- oral liquid gels
- oral liquid syrup
Zyrtec products are usually taken once daily.
Side effects and risks
Benadryl and Zyrtec have some similar side effects, and some that differ. Below are examples of these side effects.
* Both Benadryl and Zyrtec can cause sleepiness, but its more common in people who take Benadryl.
Effectiveness
Benadryl and Zyrtec are both effective for reducing symptoms of hay fever and other allergies, and for treating hives or itchy skin. However, Benadryl isnt usually a first choice for these conditions because of its risk of side effects such as sleepiness. Zyrtec and other second-generation antihistamines are usually preferred.
Costs
Benadryl and Zyrtec are both brand-name OTC products. Zyrtec usually costs more than Benadryl.
Both of these products have store-brand versions. Store brands are usually cheaper than the brand-name versions.
Benadryl vs. Allegra
Benadryl is a first-generation antihistamine. Allegra (fexofenadine) is a newer, second-generation antihistamine. Second-generation antihistamines are often called nonsedating antihistamines because theyre less likely to cause sleepiness than first-generation antihistamines.
Both Benadryl and Allegra are OTC medications.
Uses
Oral Benadryl products are approved for decreasing symptoms of hay fever and other respiratory allergies, and symptoms of the common cold, such as sneezing and runny nose. Topical Benadryl products are approved for decreasing pain and itchy skin due to hives, insect bites, and other causes.
Allegra is approved for decreasing symptoms of hay fever and other respiratory allergies, and itchy skin caused by hives, insect bites, and other causes.
Drug forms
Benadryl is available in many different forms, including:
- oral tablets
- oral liquid-filled capsules (liqui-gels)
- oral chewable tablets
- oral liquid solution
- topical cream
- topical gel
- topical spray
- topical stick
Oral Benadryl products are usually taken every 4 to 6 hours. Topical Benadryl products are typically used up to four times daily.
Allegra is also available in many different forms, including:
- oral tablets
- orally disintegrating tablets (meltable tablets)
- oral gel-coated tablets (gelcaps)
- oral liquid suspension
Allegra products are taken once or twice daily.
Side effects and risks
Benadryl and Allegra have some similar side effects, and some that differ. Below are examples of these side effects.
Effectiveness
Benadryl and Allegra are both effective for reducing symptoms of hay fever and other allergies and for treating hives or itchy skin. However, Benadryl isnt usually a first choice for these conditions because of the risk of side effects such as sleepiness. Allegra and other second-generation antihistamines are usually preferred.
Costs
Benadryl and Allegra are both brand-name OTC products. Allegra usually costs more than Benadryl.
Both of these products have store-brand versions. Store brands are usually cheaper than the brand-name versions.
Benadryl vs. Unisom
Benadryl contains the ingredient diphenhydramine, a first-generation antihistamine.
There are different forms of Unisom. Most of these also contain the ingredient diphenhydramine. However, one Unisom product contains a similar drug, doxylamine.
Uses
Oral Benadryl products are approved for decreasing symptoms of hay fever and other respiratory allergies, and symptoms of the common cold, such as sneezing and runny nose. Topical Benadryl products are approved for decreasing pain and itchy skin due to hives, insect bites, and other causes.
Although its not approved for this purpose, some people take Benadryl to help improve their sleep. Unisom is approved for helping relieve occasional sleeplessness. Its not intended to be used to treat ongoing or long-term insomnia.
Drug forms
Benadryl is available in many different forms, including:
- oral tablets
- oral liquid-filled capsules (liqui-gels)
- oral chewable tablets
- oral liquid solution
- topical cream
- topical gel
- topical spray
- topical stick
Oral Benadryl products are usually taken every 4 to 6 hours. Topical Benadryl products are typically used up to four times daily. Oral Benadryl products are not approved for sleeplessness, but some people take oral Benadryl once before bedtime for that purpose.
There are also several forms of Unisom products. These include:
- diphenhydramine-containing products:
- oral softgels (SleepGels)
- oral mini capsules (SleepMinis)
- oral liquid
- orally disintegrating tablets (SleepMelts)
- doxylamine-contain product:
These products are usually taken once daily, just before bedtime or at bedtime.
Side effects and risks
Benadryl and most Unisom products contain the same ingredient, diphenhydramine. One form of Unisom contains a different ingredient, doxylamine. Doxylamine is very similar to diphenhydramine and causes very similar common and serious side effects.
The most common side effects of Benadryl and Unisom include:
Some serious side effects can include:
Effectiveness
Benadryl and most forms of Unisom contain the same active ingredient, diphenhydramine. Both products can help people with occasional sleeplessness to fall asleep. This effect may decrease or wear off with continued use.
According to a
Costs
Benadryl and Unisom are both brand-name OTC products. These products usually cost about the same.
Both of these products have store-brand versions. Store brands are usually cheaper than the brand-name versions.
Benadryl vs. melatonin
Benadryl contains the ingredient diphenhydramine, a first-generation antihistamine.
Melatonin is a hormone that naturally occurs in the body. Its involved in regulating the wake-sleep cycle of the body. Its available as a dietary supplement.
Uses
Oral Benadryl products are approved for decreasing symptoms of hay fever and other respiratory allergies, and symptoms of the common cold, such as sneezing and runny nose. Topical Benadryl products are approved for decreasing pain and itchy skin due to hives, insect bites, and other causes.
Although its not approved, some people take oral Benadryl to help improve their sleep.
Melatonin is most commonly used to help relieve sleeplessness.
Drug forms
Benadryl is available in many different forms, including:
- oral tablets
- oral liquid-filled capsules (liqui-gels)
- oral chewable tablets
- oral liquid solution
- topical cream
- topical gel
- topical spray
- topical stick
Oral Benadryl products are usually taken every 4 to 6 hours. Topical Benadryl products are typically used up to four times daily. Oral Benadryl products arent approved for sleeplessness, but some people take oral Benadryl once before bedtime for that purpose.
Melatonin is also available in different forms, including:
- oral tablets
- oral gummies
- orally dissolving tablets (fast-dissolving tablets)
- oral capsules
- oral chewable tablets
- oral liquid
Melatonin is usually taken once daily at bedtime.
Side effects and risks
Benadryl and melatonin have some similar side effects, and some that differ. Below are examples of these side effects.
Effectiveness
Benadryl can help people with occasional sleeplessness to fall asleep. However, this effect may decrease or wear off with continued use of the product.
A
According to the
Costs
Melatonin usually costs more than Benadryl.
Both of these products have store-brand versions. Store brands are usually cheaper than the brand-name versions.
