Vitamin D supplements to stop a cold before it starts & is also critical to proper functioning of your immune system. Being deficient in Vit D increases your risk of catching a cold or flu by 50%. Have you dr. check your Vit D levels with a simple blood test to see if you need a supplement. Usual dose is 1000-5000 mg/day for adults. $15 for std size bottle of Vitamin D supplements. It might not hurt to take this supplement per bottle's directions during cold & flu season.