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A 38-year-old member asked:

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

3 doctor answers5 doctors weighed in
Dr. Stephen Kimura
Allergy and Immunology 35 years experience
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.
Dr. Jack Mutnick
Allergy and Immunology 17 years experience
Antihistamines: 1st generation are better antihistamines but have significant drowsiness. 2nd generation do not bind the receptor as well as 1st generation, but they cause much less drowsiness.
Dr. Manav Singla
Allergy and Immunology 23 years experience
Sleepyness: The first generation antihistamines (benadryl, chlor-trimeton, tavist) are short acting and usually make people sleepy. Second generation (claritin, zyrtec, allegra, xyzal, (levocetirizine) clarinex) are long acting once a day, and generally do not make people sleepy, or less so.

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Similar questions

A 30-year-old member asked:

Is there an over-the-counter antihistamine medication available to treat poison oak?

2 doctor answers3 doctors weighed in
Dr. Barbara Stark Baxter
Allergy and Immunology 42 years experience
Steroids needed: If you have a significant case of poison oak, antihistamines are unlikely to faze the itching. You probably need oral steroids. You can certainly try oral antihistamines and topical steroids like cortisone 10, but these rarely control the severe itch of even a moderate case. Better see your doctor for stronger stuff. Most cases last about 3 weeks.
CA
A 34-year-old member asked:

What is the highest dosage of antihistamine that one person can take at one time?

2 doctor answers7 doctors weighed in
Dr. Michael P Vaughn
Allergy and Immunology 34 years experience
Antihistamines: Safety of high dose medications is often unknown. Our experience comes from case reports of persons who intentionally or accidentally consumed unusually excessive quantities. When Claritin (loratadine) was first investigated, safety studies were done at 40 mg (4 times the current recommended dose). We take 10 mg because effectiveness is not significantly different between 10 mg and 40mg.
A 27-year-old member asked:

Why is it that Benadryl (diphenhydramine) gives me a hangover?

2 doctor answers4 doctors weighed in
Dr. Stephen Kimura
Allergy and Immunology 35 years experience
Crosses into brain: There is a blood:brain barrier and medications can cross through this barrier to cause people to feel "hungover". People complain of feeling drowsy or paradoxically some people get insomnia. Non-sedating antihistamines (fexofenadine, loratidine, cetirizine) have less of this "crossover" and so cause less problems for people.
A 48-year-old member asked:

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

1 doctor answer3 doctors weighed in
Dr. Richard Wasserman
Allergy and Immunology 44 years experience
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.
A 36-year-old member asked:

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

2 doctor answers11 doctors weighed in
Dr. Al Hegab
Dr. Al Hegabanswered
Allergy and Immunology 40 years experience
Sedation & tolerance: most first generation antihistamines cause sedation and induce tolerance, meaning that if you keep taking them, then you would need higher doses after sometime to have the same clinical effect, that's not the case with most second or third generation antihistamines

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Last updated Jan 28, 2019

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