Why a soothing throat may not quiet the lungs
A cough is a protective reflex: nerves detect irritation and send a signal that triggers a forceful burst of air. Those nerves are found in more than one place, including the throat and the airways inside the chest. A cough drop mainly acts on the sensations near the mouth and throat, so it may ease a scratchy feeling without silencing signals that began deeper down.
What menthol changes
Menthol activates cold-sensitive nerve endings in the lining of the mouth and throat. The resulting cooling sensation can make irritation feel less noticeable for a while. That is a change in how the area feels, not necessarily a change in what is provoking the cough. Menthol does not travel down the airways to coat or switch off the source of a lower-airway cough.
Why the cough can continue
If irritation in the lower airways keeps activating cough-sensitive nerves, the reflex can persist even while the throat feels calmer. A drop may still make swallowing or talking more comfortable, but it cannot be expected to address every cause of coughing. Relief at the throat and relief at the source are not always the same thing. That is why a minty, soothed throat can coexist with a deep cough that keeps returning.
