How much can antihistamines affect result of pregnancy test?

It does not: An anti-histaminic does not affect the results of a properly done pregnancy test.

Not at all: Antihistamines do not affect the result of either urine or blood pregnancy tests. Hope this helps!

One in particular: There's one antihistamine in particular, called promethazine, which is also sometimes used for nausea and in cough medications, which can affect the result of a pregnancy test. The other more common ones like Claritin, (loratadine) Zyrtec, and Allegra should not affect a test.