Taking 13 milligrams every three to four hours during the day, for a total of 50-65 milligrams per day, is what seemed to cut down colds.
You only suffer 60% of the normal days of coughing/sneezing when you take zinc on the first day (avoid nasal spray zinc). Source: Cochrane Database of Systematic Reviews I’ve weakly believed in this for over 10 years, but haven’t consistently employed it. Also (not sure of research), many shut-ins or people in low-sunlight areas claim that taking 10x the RDA of vitamin D has resulted in their not succumbing to colds. This is quite weak anecdotal evidence.