It's not a medicine.

However, like a number of liquids, it has a reputation of being comforting and settling a queasy stomach. Some others are tea (when I was sick my mother always brought me "tea and toast"), ginger candies (sometimes sold in the U.S. at excursion-cruise docks to help combat motion sickness), and of course chicken soup. 

My mother did sometimes offer us carbonated drinks when we were sick. Sweet, slightly tart, syrupy drinks help, and, oddly enough, the carbonation seems to help, too.

Then, too, Coca-Cola (like tea) has a small amount of caffeine in it--much less than coffee--and that generally helps people feel better.

My understanding is that in some parts of the country it was or perhaps still is traditional to use Coke syrup, the syrup that is diluted to make the drink, as a kind of informal medicine. 

Very early in its history Coca-Cola was sold as a kind of medicine, a "tonic." It was fairly common during that era, and some drinks sold as medicines later became drunk just because people liked the taste. The only medicinally active ingredient in Coca-Cola was cocaine, but very quickly the cocaine was first reduced to a trace, then eliminated altogether.

When I was a child, this was a typical treatment when we were sick. Our parents would give us flat lemonade.*

I don't think there's anything especially medicinal in Coke, seeing that it's mostly sugar and a little flavour. Perhaps it is the flavour that helps as well. I think this treatment is for gently rehydrating and providing a little energy in the form of sugar. 

* In Australia, lemonade is what the rest of the world would call Sprite. ;) For the yellow lemony-flavoured softdrink, we call it lemon squash, or we use a brand name, Solo.

Hey Maggie, 

Yes, in some gastric cases it helps indeed, mainly because of a very high content of phosphoric acid. It erodes fat deposits, rust and stein. That is why the coke is perfect for descaling. I use it to clean my toilet and I recommend to do the same at your house.