Would taking allegra (fexofenadine) before a methacholine challenge effect the results?

It can. You should obviously avoid asthma inhalers. The longer the work, the longer you should stop them for (eg advair for 24hrs). Antihistamines may not have as big an impact on the test as inhalers, but they should also be avoided for 24-48hrs.

Related Questions

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. Read more...