But there are studies that high doses can help a lot with colds.
EDIT:
examples:
http://www.mdpi.com/2072-6643/9/4/339
https://www.eurekalert.org/pub_releases/2017-03/uoh-ldo03301...
https://www.consumeraffairs.com/news/high-doses-of-vitamin-c...
After reading about it i tried this, and confirm that high doses help to cure colds much faster. In 2 of three cases i managed to get over a cold which otherwise lasts 7-10 days for me in just 3-4 days with much lower symptoms. The dose at which this begins to work is 3g (yes 3000mg!) per day - i take 6x1g the first day, then 3g/day a few days more. Pure Vitamin C powder with no other additives(many products nowadays are combinations, in spain where i am right now, the pure powder is hard to get, in Germany it’s in every drugstore). It works even better with herpes i sometimes get on the lips. Here, the success rate of making the illness last 2-3 days instead of 10 is 100% since i started do treat it that way a few years ago, with much less symptoms, just a little itching, no big wounds for many days like many suffer from (and i did) when getting a herpes push.
I can clearly understand why this is not researched and communicated much - a pot of 100g Vitamin C powder, good for about 5 such treatments costs just 3€ ... not much money to be made.
EDIT:
This is my personal experience - for which I don't claim general applicability for everyone. But I find it very interesting that some people see the urgent need to downvote a post where someone reports personal experience, without saying it must be generally applicable.