Vitamin C

Vitamin C as a dietary supplement is viewed by science (through randomized trials, not observational) as likely to INCREASE the risk of dying. So might do the opposite of it aims to do on the label. In other words, those who take Vitamin C, might die 6% more based on the scientific randomized study below (although the result for Vitamin C was not statistically significant).

Vitamin C supplement use is still discouraged by scientists behind these studies.

Here's a more detailed video on that study discussion from the University of California San Francisco Department of Medicine:

https://youtu.be/2mDrAQi1SwU?t=1146 

Using Vitamin C as a cold remedy

There's evidence that Vitamin C reduces the duration of a cold by 10% (with additional benefits for marathoners, skiers, etc. - see below)