Antihistamines block histamine reactions in the body - runny nose, sneezing, watery eyes. More often antihistamines are used for allergies such as hay fever, but because some of the symptoms are similar, they may provide some relief from cough and cold symptoms. The non sedating antihistamines are ideal for day time use when you need to be alert and the sedating one for bedtime use when you want to sleep uninterrupted by a drippy nose. Decongestants alleviate congestion enabling you to breath better. Because sinuses often become congested with colds, decongestants are found in many sinus products.
Decongestants are available as both tablets and nasal spray.
The nasal sprays are associated with the risk for rebound congestion - you have to keep using increasing doses of the spray to keep your nasal passages open. People with certain medical conditions need to use decongestants with care, for example blood pressure control can be affected. To control coughs antitussives or cough suppressants are included in many formulations. Drowsiness can be a sided effect and because DM works through the nervous system to stop your cough, it can interact with other drugs that affect the nervous system, for example antidepressants. Sometimes, in the case of a productive cough, you want to liquefy phlegm in order to cough it up more effectively.