What is the best decongestant for an allergic cough?

Nasal steroid but. Nasal congestion is not the cause for an allergy cough but post-nasal drips and/or asthma may as would be a sinus infection. Nasal steroid remains the most effective if post-nasal drips are the cause for the cough. You may need to consult a doctor re. the cough should nasal steroid fail or the problem gets worse.