★★★★★
My son had several bad coughs (bronchitis) within a few months, and colds almost every month, so I started giving him vitamin C daily - usually about 4-5 grams a day in 2-3 doses. He didn't have a cold for 7 months, and so far didn't get any serious cough.
When we did have coughs, we used herbs - I have lots of tinctures I buy from Oregon online store or make my own with brandy (vodka just tastes very bad even in small amounts), and make mixtures using my intuition, but the main ones were wild cherry bark, mullein, Echinacea, Umckaloabo (Pelargonium sidoides), and lobelia. We also used propolis with good results, however it tastes not so good. We would take 2-3 droppers of mixture of several of these. It works within 10-15 minutes, so if it doesn't stop coughing in that time, another dose might be needed. But usually it did and he could go back to sleep at night.
There is another herb, that is very effective, but it tastes bad -like a sap or a paint thinner - lomatium (lomatium dissectum). Once I kicked the cold with it -although I had to take a few more doses every 4 hours to kick it for good. That said, I get one cold a year at the most, so I am not sure it would work for everyone. I take vitamin C and other supplements for other reasons, which I think helps to prevent them.
