Pseudoephedrine may cause mild central nervous system stimulation, especially in those patients who are hypersensitive to sympathomimetic drugs. Nervousness, excitability, restlessness, dizziness, weakness and insomnia may also occur. Headache and drowsiness have also been reported. Large doses may cause lightheadedness, nausea and/or vomiting. Sympathomimetics have been associated with certain untoward reactions including fear, anxiety, nervousness, restlessness, tremor, weakness, pallor, respiratory difficulty, dysuria, insomnia, hallucinations, convulsions, CNS depression, arrhythmias, and cardiovascular collapse with hypotension. Adverse effects associated with dextromethorphan are generally infrequent and mild. Products containing dextromethorphan have been associated with nausea, dizziness, fatigue, gastrointestinal disturbances, and skin eruptions.
Chlorpheniramine may cause slight to moderate drowsiness and is the most frequent side effect. Other possible side effects of antihistamines include:
General: Urticaria, drug rash, anaphylactic shock, photosensitivity, excessive perspiration, chills, dryness of mouth, nose and throat;
Cardiovascular: Hypotension, headache, palpitation, tachycardia, extra systoles;
Hematological: Hemolytic anemia, thrombocytopenia, agranulocytosis;
CNS: Sedation, dizziness, disturbed coordination, fatigue, confusion, restlessness, excitation, nervousness, tremor, irritability, insomnia, euphoria, paresthesia, blurred vision, diplopia, vertigo, tinnitus, hysteria, neuritis, convulsion;
Gastrointestinal: Epigastric distress, anorexia, nausea, vomiting, diarrhea, constipation;
Genitourinary: Urinary frequency, difficult urination, urinary retention, early menses;
Respiratory: Thickening of bronchial secretions, tightness of chest, wheezing and nasal stuffiness.