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Doctor insights on: 1st Generation Antihistamines

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What is the reason behind 1st generation antihistamine induce sleep ( by which mechanism cause sleepiness in our brain ) ?

What is the reason behind 1st generation antihistamine induce sleep ( by which mechanism cause sleepiness in our brain ) ?

Anticholinergic: First generation antihistamines block the h1 and h2 histamine receptors but also have some anticholinergic effects by partially blocking other receptors. This can lead to dry mouth, urinary retention, sedation and other side effects. This is usually dose dependent and worsens with high doses. ...Read more

Benadryl (Definition)

Diphenhydramine is a sedating antihistamine which is a kind of antihistamine type drug (common ...Read more


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What is the difference between first and second-generation antihistamines?

What is the difference between first and second-generation antihistamines?

Side effects: The second generation antihistamines were developed to try and decrease the side effects of the first generations. Many are "purified" first generation antihistamines so they have the active effects without the side effects of drowsiness, sedation, dry mouth etc. They also tend to have longer half lives, meaning they can be taken once or twice a day not every 4 hours. ...Read more

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Difference between first and second generation antihistamines? Why are 1st generation considered nonsedating?

Difference between first and second generation antihistamines? Why are 1st generation considered nonsedating?

Sedation: 1st generation are sedating because many people, up to 40%, get sleepy when taking them. 2nd generation antihistamines are chemically modified and are less likely to cause sedation. The generics of the less sedating are cetirizine, fexafenadine, and loratidine. They cause sedation in some people, up to 11%, but much less than the older drugs. ...Read more

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Are first generation antihistamines more effective than second generation? I always wonder why some are still on drugstore shelves, like chlorpheniram

Are first generation antihistamines more effective than second generation? I always wonder why some are still on drugstore shelves, like chlorpheniram

Less side effects: There are many medications that remain on shelves for over-the-counter use that probably should be removed. New generation antihistamines are as effective as old ones. Of interest is that some individuals respond favorably to one over the other. The side effect profile and frequency of dosing is what really separates the new from the old antihistamines. In this case, newer is better. ...Read more

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Doctors, what is the difference between first and second generation antihistamines?

CNS: The 1st generation AH gets into the brain area thereby blocking the stimulatory effect on alertness from histamine. The 2nd generations ones do not get into the brain and thus do not usually induce drowsiness. 1st gen- more drying and better for runny nose , the sedating effect also better for itching than 2nd gen. ...Read more

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Whats the difference between a first generation antihistamine like chlorpheniramine and second generation antihistamines like allegra (fexofenadine)?

Whats the difference between a first generation antihistamine like chlorpheniramine and second generation antihistamines like allegra (fexofenadine)?

1st and second: First generation antihistamines are sedating and short acting and have usually other properties such as anticholinergic or drying properties. Second generation antihistamines are long aciting, usually dosed once a day, and are less or non sedating and selective antihistamines with less active properties on other receptors. ...Read more

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Is antihistamines first generation effective drug for common cold and does vitamin c important during common cold?

Probably not: Antihistamines are effective for sneezing and runny nose due to allergies, by blocking histamine receptors,. but can over dry nasal passages during colds. Atrovent nasal spray helps runny nose and decongestants (use Short term only) help congestion. Vitamin C may help prevent colds, but not as effective once you have one. Rest, fluids, chicken soup, zinc, humidifier, and netipot help. ...Read more

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I am about to take an antihistamine for the first time but my friends tell me to use it before bedtime. Will I still feel the effects the next day?

Depends on the med: Some antihistamines are more sedating than others. Medications like Benadryl (diphenhydramine) can definitely cause sedation. Newer antihistamines like Allegra and Claritin are non-sedating and Zyrtec is sedating in roughly 10-20% of patients. ...Read more

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2 days of hive outbreak has gotten worse - unsure what caused it but this is not the 1st time. Been taking off-brand antihistamine to no effect. Help!

2 days of hive outbreak has gotten worse - unsure what caused it but this is not the 1st time. Been taking off-brand antihistamine to no effect. Help!

See a dermatologist: May be allergy. Maybe not. Zyrtec (cetirizine) (cetirazine) OTC 10mg. May need 2 or 3. Larger dose can make you sleepy. If severe, add prescription hydroxyzine 25mg at bedtime. If present over 6 weeks it is chronic urticaria which is usually not allergy. Avoid aspirin, alcohol. Beware of any and all pills or meds of any kind. Need precise diagnosis for best result. ...Read more

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What if I am concerned my hives are caused by an antihistamine in the first place? I.E. Zyrtec and allegra (fexofenadine)

Go to allergy dr: Go to an allergy doctor and get tested. Usually if you take antihistamine and break out more you might suspect a reaction to antihistamine, but you won't know for sure unless it happens every time you take it, or you get an allergy testing. ...Read more

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Dr. Paul Carter
601 doctors shared insights

Antihistamine (Definition)

Sedating and non-sedating antihistamines prevent and treat allergic reactions, and are ...Read more