Only certain human over the counter cough medications are safe to use for dogs with dry coughs, and then only in small dosages. Before using human cough medicine for dogs, please contact a vet. According to "Dog Owner's Home Veterinary Handbook" (Debra M. Eldredge, DVM, et al, 2007), the active ingredient in human cough syrups that works for dogs is dextromethorphan.
Brands that contain dextromethorphan include Robitussin DM, Benylin Expectorant, Hold, Babee Cof, St, Joseph Cough Suppressant For Children and Vicks Formula 44 Soothing Cough Relief. There are also store brands of over the counter medications that use dextromethorphan as the active ingredient. These brands often come in tablet form as well as liquid.
Weigh the dog. According to "Dog Owner's Home Veterinary Handbook" the average dose is one teaspoon of medication for every 9.07 Kilogram that the dog weighs. This works out to 0.5 to 1ml. per lb. or 1 to 2mg. per kg. that the dog weighs. However, your vet may want to give a smaller dose, such as 0.25mg. per lb. that the dog weighs. Doses are given every six to twelve hours.
Dextromorphan is not approved for use in dogs. This is an off-label use of the medicine. Sometimes it can be mixed with other ingredients that are not good for dogs. Do not use if the cough medicine contains acetaminophen, paracetamol, caffeine or alcohol. The ingredient guiafenesin is safe. Don't give to pregnant or nursing dogs and do not give to dogs that have heart disease, are coughing up mucus or have a collapsed trachea.
According to "The Pill Book Guide to Medication For Your Dog and Cat" (Kate Roby, VMD, et al, 1998), symptoms of overdose include vomiting, hyperactivity, dizziness, sudden drowsiness and breathing problems. Contact the vet immediately if the dog has one or all of these symptoms.
Dextromorphan works by suppressing the dog's coughing reflex, helping lessen irritation in the throat and hopefully speed healing. Unlike many other kinds of cough medications for dogs, it does not normally make a dog very sleepy, unless there is an overdose. If the medication also has guiafenesin, then the latter medication helps to break up any minor congestion. However, since dextromorphan is given to dogs with dry coughs, such as with kennel cough or chronic bronchitis, congestions should not be a problem.