Ascorbic acid (vitamin C) occurs naturally in foods such as citrus fruit, tomatoes, potatoes, and leafy vegetables. Vitamin C is important for bones and connective tissues, muscles, and blood vessels. Vitamin C also helps the body absorb iron, which is needed for red blood cell production.
More info »
Regarding the effect: I do notice an increase in bloodflow and a better stamina using the pills (I use 50mg). A side-effect I have noticed is that my nose clogs up so I seem like I have a cold for an hour or so after taking one pill. No other side-effects than that.