First off, it’s important to know that these medications are no longer prescribed or suggested for children under the age of two. Studies have shown that they can cause side effects that are potentially life threatening. Never give an expectorant or a cough suppressant to a child younger than two.
For children over age two, an expectorant will help to loosen secretions so that the cough is more productive, while a cough suppressant will suppress the need to cough.
For both expectorants and cough suppressants however, there is no solid medical evidence that either are effective. In general, cold remedies, whether they are over the counter or prescribed, cannot cure a cold or even lessen the duration of a cold. These products are only intended to alleviate some of the symptoms of a cold.
Many doctors advise parents to use a vaporiser or humidified to loosen secretions instead of using an expectorant or a cough suppressant.