Vitamin C can slightly shorten a cold, but only if you take it every day.

The study linked in the post shows that regular vitamin C supplementation slightly shortens colds by about 8 to 14 percent, but taking it after you're sick doesn't help much. Other research confirms that high doses can make colds less severe, but the effect is modest. As for zinc, the linked study suggests it can help the immune system fight viruses, but it's unclear if it actually shortens cold symptoms. Some studies found no benefit for vitamin C when taken after symptoms start, while others show a small improvement. The overall picture is that vitamin C helps prevent or slightly shorten colds if taken daily, especially for people under physical stress, but it's not a cure-all.