What is Robitussin
Robitussin is an over-the-counter cough and cold suppressant. The active ingredients are either guaifenesin or dextromethorphan, often in combination with secondary components like acetaminophen, codeine and pseudoephedrine [4, 7, 8].
Can you give your dog Robitussin
The two main formulations of Robitussin easily available are Robitussin DM and Robitussin AC. Vets usually avoid prescribing the AC variant, as its codeine component is not safe for dogs. But, Robitussin DM [generic name(s): guaifenesin/dextromethorphan HBr] is often prescribed for coughs stemming from a collapsed trachea or dry kennel coughs.  . It is considered reasonably safe for both smaller and larger dogs.
However, the FDA has not approved either the DM or AC variant for use in animals .
How does Robitussin DM work in dogs
Guaifenesin is an expectorant that works by reducing mucus obstruction in the airways, loosening chest congestion , while dextromethorphan is a cough suppressant that works by blocking cough signals from reaching the brain . Robitussin can come in the form of tablets, cough gels, and liquid syrups.
How much Robitussin DM to give a dog
Never give more than the prescribed dose. Usually it is within 1mg/lb every 8 hours; however, this can vary based on the cause and severity of the cough, as well as the age and overall health of your pet .
Signs of overdose
Overexcitement or confusion, with or without labored breathing, is a possible sign of overdose. 
Immediately contact your vet or call the Animal Poison Control Center Helpline (1-888-426-4435).
- Vomiting 
Call your vet as soon as possible in case of any of these side effects.
When not to give Robitussin to your dog
Robitussin DM is not recommended for dogs suffering from pneumonia, or if the cough is moist and producing phlegm. 
It is usually not prescribed within 14 days of using a MAO inhibitor, or alongside other sedatives like tranquilizers, antihistamines, narcotics or CNS depressants.