Apart from herbal syrups, most cough medicines contain cough suppressants contain pholcodine or dextromethorphan. These compounds tell the brain to stop the coughing reflex. Therefore, they can make you dizzy and sleepy. Well, this is fine if you are staying in bed. However, this isn’t good if you’re driving or performing any other important works. Highly unlikely, they can also make you sleepy, hallucinate and give you palpitations.