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Scott_Abrams t1_iyqgkf5 wrote

The simple answer is yes but the more complex answer is maybe.

To understand what an antihistamine is, you have to first understand what a histamine is. A histamine, in simplest terms, is a compound involved in local immune responses and a neurotransmitter. It is this compound which triggers many allergic reactions, mainly by binding a cell's histamine receptor. There are 4 types of histamine receptors, aptly numerated by H1-H4.

An antihistamine is a classification of drugs which counters the activation of a cell's histamine receptor by blocking the receptor without triggering a response (receptor antagonist). Though it is commonly understood that antihistamines are used in the prevention/treatment of allergic reactions, an antihistamine is not synonymous with anti-allergy drugs as there are drugs out there which treat allergies/inflammations which are not triggered by histamines, such as corticosteroids (antihistamines do not work on non-histamine allergies/inflammations). Silver allergies for example, is not a histamine allergy and is treated by corticosteroids.

You mentioned that you're taking levocetrizine, which targets H1 receptors. It would not be effective if you had allergies on H2-H3 or any other non-histamine allergy triggers. Mosquito bites, in particular the itching reaction, is strongly correlated to be affected by the activation of the H1 receptor. By taking levocetrizine, you've blocked the H1 receptor and thus, have suppressed the itch activation.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3684411/

Thus, to answer your question, first of all, it would depend on which type of antihistamine you're taking (which type of histamine inhibition, from H1-H4) to see what other potential allergic/inflammatory reactions you're also suppressing). A H1 inhibitor antihistamine will not work on a H2-H4 histamine reaction or vice versa (it must be targeted). Secondly, antihistamines will only work on allergies/inflammations caused by histamines - it is not a cure-all for allergies as there are many different types of allergens (such as silver). Lastly, you are correct in identifying that taking an antihistamine for one allergen may reduce the effect of another potential allergen if they are both triggered via similar pathway. However, you should not preemptively self-medicate without proper medical guidance as each allergic reaction and subsequent treatment should be taken by a case by case basis. Also, taking antihistamines in the long-term can be detrimental to your health so please take care when you're using drugs and only use drugs in the manner prescribed.

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reddit11235813 t1_iyqj69t wrote

Are you aware of any studies done to understand effects of daily cetirizine use over long periods ?

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[deleted] t1_iyr4jzq wrote

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UniqueLoginID t1_iyqhfx7 wrote

In what way are they detrimental to your health if taken long term, daily?

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[deleted] t1_iyqjux1 wrote

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[deleted] t1_iyqojhy wrote

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[deleted] t1_iyqiw66 wrote

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[deleted] t1_iyrjd4z wrote

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JackMasterOfAll t1_iyrs76d wrote

Those side effects are from 1st generation antihistamines because they cross the blood brain barrier. To my knowledge loratadine and cetirizine do not cause those since they are second generation and do not cross the BBB. That said there should still be some side effects but shouldn’t be that significant.

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supervisord t1_iyry3ql wrote

So Zyrtec, for example is cetirizine and can be taken daily without all of those nasty side-effects? Nice.

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hiv_mind t1_iys33w1 wrote

Sort of. It's a "lesser-sedating" antihistamine, compared with first generation. But it's one of the most sedating of its generation. Loratadine is better, and fexofenadine better still (ref: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1118461/).

With "lesser-sedating" antihistamines, it's just a less favourable concentration gradient to get it through the BBB - so higher doses absolutely can get central effect.

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[deleted] t1_iys0kp2 wrote

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Much_Elephant t1_iysgbbc wrote

> but I wouldn’t take long term anything (daily for many months to years) unless medically necessary.

Nobody is taking daily allergy medication when it's not medically necessary.

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JackMasterOfAll t1_iyshcf5 wrote

Contrary to your belief, many people take medications for things like ultra mild rhinitis when it’s not medically necessary.

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redskea t1_iys5ruh wrote

Zyrtec makes me really drowsy but Loratadine is fine. I’ve got lots of weird drug allergies and reactions though.

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AaronfromKY t1_iys9aye wrote

Xyzal puts me out super quick, so I can't even take it during the day. Definitely helps me get sleep most nights.

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Much_Elephant t1_iysgm6v wrote

Xyzal is just the better half of Zyrtec so that makes sense. Zyrtec contains both levo and dextroceterizine and dextroceterizine isn't useful. Xyzal is the levoceterizine portion alone.

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mr-rob0t t1_iysgxm7 wrote

These are some tall claims without a source cited. Can you provide a source?

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[deleted] t1_iyqi4d0 wrote

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Scott_Abrams t1_iyqjo0l wrote

The majority of histamine allergies are on H1 and H2. H3 and H4 are pretty rare. H1 are where most allergic reactions take place while H2 is the gastric stuff. I imagine if you're taking H1, most of your other histamine allergies will also be impacted. H3 is mostly found in the brain and H4 is kind of special. H4 was found while combing through the human genome and it seems to affect the shape of some white blood cells and the movement of mast cells. H4 is not well understood as it was only discovered in 2000. H4 receptor antagonists are being studied for the potential treatment of asthma.

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[deleted] t1_iyrocp7 wrote

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gauchocartero t1_iyqwj16 wrote

Thank you for the detailed answer. I had a question regarding mirtazapine, an atypical antidepressant which is useful to treat insomnia because of its extremely high affinity to H1 receptors (Ki = 0.14nM). Essentially at lower doses mirtazapine binds exclusively to H1 instead of adrenergic and serotonin receptors. Would mirtazapine treatment upregulate/sensitise H1 over time, reducing its effectiveness as a sedative and potentially increasing the risk of allergies?

I’m also curious about the molecular mechanisms by which tolerance to antihistamines develops. IIRC antagonists and inverse agonists inhibit the receptor, except the latter bind at the agonist site and induce an opposite response instead of blocking it. Would inverse H1 agonists hence be more likely to sensitise H1 as the body tries to return to baseline? How does the neuron achieve this? By increasing the amount of H1 or by decoupling G proteins from the receptor?

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its_justme t1_iyr6zz6 wrote

Antihistamines can also be used for acid reflux issues as H2 receptors are involved with acid production in the stomach.

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