SN instructed patient that Robitussin its used as a cough medicine, and it is sell over the counter. It contains (Dextromethorphan and guaifenesin) a combination medicine used to treat cough and chest congestion caused by the common cold, infections, or allergies. Common side effects may include: dizziness, drowsiness; sleep problems (insomnia); diarrhea; or feeling nervous, restless, anxious, or irritable. Robitussin Overdose may cause breathing problems and cardiac arrhythmia.