Ailidh
Doxycycline is great for chest infections but it's a vicious wee thing!
I had it in March, and just felt faintly gruey with it, as antibiotics can make one feel.
Got some last Tuesday: almost immediately I could feel my chest clear but by Friday I felt dreadful, and was back to coughing my head off. I gave up Saturday morning. (Discovered a cough is a known side effect).
Result! Chesty cough gone, Doxy cough gone. Sorted.
It does the job IMHO but takes no prisoners. Good luck!
Your post brought back memories of a nasty chest infection our whole household came down with back in 2013. Our GP was on the point of having me admitted to hospital but said he would try doxycycline first. I think vicious is the right word for it - it made me really sick at first, but boy did it work and I avoided a hospital stay.