**Abstract**

The dosage of any compound determines its efficacy in therapy and/or prevention of any disease. Such is also true about vitamin C. Unlike most other vitamins, L-ascorbic acid seldom leads to toxicity or an overdose. It is well-tolerated at extremely high dosages with little or no side effect. Vitamin C at different doses can act as either an antioxidant or a pro-oxidant. Either way, it is therapeutic in several conditions. In this chapter, we consider the appropriate doses used under normal conditions, for the maintenance of healthy state. The recommended daily allowances of vitamin C vary from country to country, and different organizations have varying values. Therefore, we look at some of the factors responsible for these variations and those that determine the efficacy of ascorbic acid at different doses. We also explore the different dosages used in several randomized controlled clinical trials for either treatment or prevention of certain disease conditions. The high dose used in disease conditions is to first replenish the depleted stores before improving the health status of the patients.

**Keywords:** disease, dosage, recommended daily allowance, high-dose, health, low-dose, vitamin C
