Side effects of chlorpheniramine include diarrhea, constipation, nausea, sleepiness and dizziness, according to Drugs.com. Other common side effects include dry mouth, excitability, weakness, nervousness or insomnia. Patients should seek immediate medical assistance if they experience difficulty breathing or urinating, irregular heartbeat, hallucinations or seizures.
The majority of patients who take chlorpheniramine experience sleepiness, which lessens in some patients who use the drug for extended periods, notes Drugs.com. Other side effects may intensify when patients taking the drug also take medications such as furazolidone, sodium oxybate, amitriptyline or phenytoin. Chlorpheniramine is an antihistamine that helps minimize cold, hay fever and allergy symptoms, such as sneezing, itching of the throat and watering eyes.