your allergy medications will not influence a pregnancy test but a pregnancy test taken very early could definitely be a false negative. you can repeat the test weekly for a while or have your gynecologist order a more-sensitive blood pregnancy test.
i am having signs of early pregnancy. my last cycle started dec 23 and ended dec 30. it is jan 14. don't know if i got a false neg due to taking allergy meds. i take 2 different type of allergy meds. i don't know if i should call the gyno