My daughter can't stop coughing, anyone tried an onion in the bedroom?

My 4 year old daughter can not stop coughing and can't shake this cold off - it goes for a week and then comes back. 

Been to the GP but they can't find anything wrong, and a friend told me to try an onion sliced in half in her bedroom tonight - was wondering if anyone has tried this?

Done Viks on the feet but no joy.