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Dogs can develop upper respiratory illnesses such as kennel cough and canine influenza as well as bronchitis and inhaled allergies which lead to coughs. You can read about bronchitis here:
It's possible that sneezing and nasal congestion can be due to an allergic reaction to an inhaled substance. If this is the case, Benadryl can be given to your dog, the dose is up to 2mg per pound every 8 hours.
You can read about these here.
Kennel cough is normally contracted when a dog has been boarded or kenneled or around a large number of dogs such as at a dog show, dog park or pet store. Here is a website with more information on kennel cough. http://www.thepetcenter.com/gen/kenc.html
Canine influenza is now becoming more prevalent and like it sounds it is a canine flu. Here is an excellent site on it.
You will want to monitor your dogs condition looking for colored discharge from the nose or eyes, a productive cough (coughs stuff up), stops eating or lethargy. These are signs of a possible bacterial infection as well and my require antibiotics. If your dog appears to be having a difficult time breathing, you will need to see your Vet as some dogs dog get really sick with canine influenza and need support to recover.
To help your dog breathe easier you can run a NON-medicated humidifier in the room your dog is in, or sit in a steamed up bathroom with your dog to help keep the mucous moving. Robitussin DM at a 1/4 teaspoon per 5 pounds can be used to control the cough. Dosages can be obtained at this website. http://www.petplace.com/drug-library/dextromethorphan-robitussin-dm/page1.aspx
There are other reasons for coughs such as heart problems which should be investigated if your dog is not on heartworm preventative or is an older dog. The following site goes over canine coughs.
Your dog could have a collapsing trachea. Here is a site on collapsing tracheas.
Small breeds do frequently have problems with collapsing traches. Owners of these dogs do say rubbing the throat during a coughing episode helps the severity and length of the coughing episode. This may help pin down the cause of the cough as well.
I hope you find this information helpful.