How can the soda drink "Moxie" help relieve coughing? It is recommended by a doctor to a friend.

Thank you for this interesting question!

With a little research I have found that the main "active ingredient" in Moxie soda is Gentian Root extractives. Otherwise it is pretty similar to most cola beverages.

Gentian root has has a long history as a tonic, however, it is not used for cough. It has antihelminthic (worms), antiseptic, and anti-inflammatory properties as well as fuctions as a cholagogue (stimulates the gall bladder), emmenagogue (stumulates blood flow to the pelvic organs), and febrifuge (reduce fever). It is mainly used for digestive disorders but should be avoided in people with peptic ulcers.

Gentian root is also the main ingredient in Angostura Bitters.

So, no, in my opinion, drinking Moxie soda would not help relieve coughing - at least there is no rational or mechanistic basis for this use that I can find.

One final note - especially if the cough has persisted longer than 2 weeks, I'd want to determine the cause of the cough. Cough is usually a symptom of an underlying disease process and can be related to anything from peptic ulcers, gastric reflux, asthma, bronchitis, pneumonia, and allergies to cancer. Get the cough checked out, then seek treatment for the cause.