Cough is a natural body reflex to expulse the foreign bodies that we inhaled through the normal breathing cycle. When cough becomes excessive, it usually indicates some sort of pathologies such as the throat or lung infection or airway obstruction by large objects. Principally, coughing during infection is beneficial as it can aid in expulsing airway secretion which contains pathological agents. Thus, the usage of cough suppressive drugs is unnecessary as the bouts of cough will stop once infection subsides unless the cough is disrupting your daily activities or sleep.
For your knowledge, there are 3 classes of drugs when you tell the pharmacist that you need cough medicine. They are the pharyngeal demulcents, expectorants and antitussives (cough centre suppressants). Frequently, you will see products with combinations of expectorant and antitussive in the market, which helps better in relieving cough by combined mechanisms. Pharyngeal demulcents are more commonly known as lozenges or linctuses which act by soothing the throat and reduce the nerve impulse from the inflamed throat lining, thus it is better in relieving dry cough only but not cough with a large volume of secretions.
Expectorant class of drugs can be divided into 2 which are bronchial secretion enhancers (mucokinetics) and mucolytics. Mucokinetic drugs will increase the secretions produced in the lungs and ease the expulsion of sputum by coughing without exertion. FDA has already stopped the marketing of all the mucokinetics due to their inconclusive efficacy and the effect on thyroid function if prolonged use except for Guaiphenesin which is a plant product. On the other hand, mucolytics able to thin the copious amount of lung secretions by breaking the network of fibres in viscous sputum. The most common drugs you will see at the counter in pharmacies are Bromhexine, Carbocisteine and Acetylcysteine.
Antitussive acts as its name suggested where it will directly act upon the cough centre in the brain by acting the cough threshold or peripherally in the airway to reduce the coughing nerve impulses. There are 4 classes of drugs under this category which are the opioids, non-opioids, antihistamines and peripherally acting antitussive. Opioids such as codeine will act centrally on the cough centre, indirectly causing drowsiness, suppressing breathing and constipation. Abuse liability is possible as codeine can be addictive. Conversely, non-opioids such as Noscapine, Dextromethorphan and Chlophedianol are more commonly used as they will not cause the aforementioned side effects and better-tolerated by most people as they can continue their daily activities in a wakeful manner. Antihistamines mostly used in cough due to allergic reactions with their abilities to inhibit histaminic receptors which are responsible for the cascade of allergy. Lastly, the peripherally acting drug known as Prenoxdiazine helps to desensitise the lung stretch receptors and reduce the impulse for coughing. However, this drug is not accepted widely as its efficacy is not impressive.
In conclusion, cough medicine is not just one cough medicine. With this knowledge equipped in your daily life, you will be more aware of what you are taking as cough medicine and prevent any unnecessary accidents due to the sedative effect of medicines.