How do you get ride of an itchy throat if you have alergys that caused it and medicine doesn't work?

Itchy throat. An itchy throat does not necessarily arise only from allergies, it may arise from post nasal drip, acid reflux (into the throat) or from other irritants and less common causes. The best policy is to be examined by an ear nose and throat doctor to determine whether there is reason to further treat for allergies or consider other causes to tailor a more effective treatment.
What medicine? How do you know allergy caused it? Allergy to what? See your allergist or primary care doctor.