No experience of it being caused by Fostair but sinus rinses such as Neilmed can help post nasal drip - although it is a frustrating thing to have so big sympathy there!
Good old post nasal drip! It can be a blooming nuisance and causes no end of problems if ignored. Fostair will help your lower respiratory tract but it sounds like you need something for your upper respiratory tract.
Nasal rinses are good - as in a good soak; technique is everything. Whether you choose something like Neilmed or Beconase, be patient and use as directed in order to avoid just swallowing the rinse without letting it roll around the sinuses. Be patient though as it will take time to work. Regular rinsing should start to show benefits after a week.
You may want to modify your diet while in this period of recovery. Get plenty of omega 3 rich food and avoid processed foods and omega 6. And keep the nose breathing going in order to keep the sinuses open. It can be tempting to mouth breath while the sinuses are playing up, but this will only exacerbate the problem as nose breathing helps keep our sinuses healthy.
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