Dog -- Not quite right

Question
Hi,Our dog, Pepe, is about 33 lbs and 15 years old. He has had coughing for some time, often accompanied by a lot of phlegm; last week his regular vet changed out meds to a cough suppressant -- prescription is: Guaifensen100/Dextromethrpn10mg one/per day.
Since then we have seen that  one time per day , he has gotten up and been unable to keep his balance (staggering just for few minutes)- appears to be driven to move but is very unstable and cannot handle stairs without falling; First two occurrences have been in the AM, and thrid in the evening(we have left him alone for a few hours and that happend at our return home)
Most of problem seem to be in his back legs.
What do you think could possibly be the problem and what can be done about it?

Thanks, Mirella

Answer
Mirella,

I recommend you take Pepe back to the veterinarian.  Coughing is either respiratory or cardiac.  It is possible that Pepe is having a reaction to the medication, but it is also possible that something else is wrong.  If he is not getting enough oxygen to his brain, either due to heart disease or a more serious respiratory disease, you may see similar symptoms.  It's time for more diagnostics to try and figure out what is causing the cough.  

Good Luck to both of you