Zyrtec (cetirizine) Which drug is better fora specific patient cannot be predicted. I generally try zyrtec first because it does cannot claim not sedating. A little sedation helps alleve itching. Allegra, (fexofenadine) claritn, and Clarinex are allowed to claim no sedation.
Benadryl (diphenhydramine) It helps if you know what caused the hives. Benadryl (diphenhydramine) will work the quickest but also will be out of your system the quickest and will sedate most people. If the hives are due to a recent pet exposure or food ingestion Benadryl (diphenhydramine) will be the best choice. If it is due to a recent exposure to outdoor pollens Allegra may be your best choice and for unknown ongoing hives I would try zyrtec. Variables.
What if I am concerned my hives are caused by an antihistamine in the first place? I.E. Zyrtec and allegra (fexofenadine)
Very rare. Hives usually come and go randomly. So you may confuse an outbreak with being caused by something else (though not really). So best to see an allergist.
Hives. That is rare. Especially if suspect it from claritin, (loratadine) allergra, benadry, and zyrtec. Sometimes you have to realize that we can't find a cause but there is a long list of causes.
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.
Which antihistamine is more effective at treating the symptoms of hives? Benadyrl or Allegra (fexofenadine)?
Benadryl (diphenhydramine) This have been my clinical experience, beware of the potential drowsiness.
Rare urticaria. Why does zyrtec work well for it but when adding allegra (fexofenadine) the hives come more? Is allegra (fexofenadine) cancelling zyrtec out on some gene level?
It shouldn't. Zyrtec and Allegra (fexofenadine) work in similar fashion and actually the result should be additive. I suspect that you added Allegra (fexofenadine) because the hives are already getting worse. It takes about a hour before most oral drugs can get into your system.
Will anti histamines such as zyrtec or allegra (fexofenadine) so on, effect methacholine since its a histmane and cause negative results?
It may potentially. Antihistamines are not to be taken for at least 24 hrs but preferably 72 hrs prior a methacholine challenge. Having said that, I would doubt the average dose of AH is potent enough to block a truly positive response.