Dry Cough and Allergies

Hey ladies...I'm dying here. I have an absolutely horrible dry cough that will not go away and will not let me sleep. It's to the point where I throw up. I have horrible allergies and asthma. My OB said it was fine and recommended that I continue taking my Singulair because of my asthma, but its just not cutting it. My inhaler and Benadryl have been added to the equation but nothing is working. I also have allergies to things like Robitussin, Mucinex, and the newer Vicks cough medicine. Any kind of chest decongestant is a no-go for me, but I'm not congested anyway. So I'm at a loss and dying for some relief. Help?!