Like all medicines, Robitussin Mucus Cough and Congestion Relief can cause side effects, although not everyone experiences them. Side effects with guaifenesin or pseudoephedrine are rare, but some individuals may experience unwanted reactions. If you experience any of the following serious effects, stop taking this medicine immediately and contact your doctor or pharmacist: allergic reactions, such as anaphylaxis, itchy skin or rashes, difficulty breathing, faster heart rate, swelling of the face or throat, and feeling faint, which may lead to loss of consciousness; a feeling that your heart is beating too fast or palpitations; sudden onset of fever, reddening of the skin, or many small pustules (possible symptoms of Acute Generalized Exanthematous Pustulosis – AGEP); inflammation of the colon due to insufficient blood supply (ischemic colitis) with symptoms like sudden abdominal pain or rectal bleeding. For a full list of side effects refer to the Patient Information Leaflet.