Hopefully, you have read about Some Potential Risks of Vitamin C. If so, you know that Vitamin C has definitely not turned out to be the rock star that so many promised. In spite of that, Vitamin C, in some cases at megadose levels, does merit very strong consideration.
Here are some of the nice benefits from Vitamin C, including some potentially significant help in the bedroom:
1) Intercourse Frequency. Yes, you read that right. One study of healthy young volunteers found that 3 grams/day significantly increased their frequency of sexual intercourse. Not a bad day's work for a cheap supplement, eh?  The authors of the study noted that vitamin C improves "catecholaminergic activity, decreases stress reactivity, approach anxiety and prolactin release, improves vascular function, and increases oxytocin release". Of course, all of these can improve your bedroom performance, desire and enjoyment.
2) Your Brain. So is there anywhere where megadoses of Vitamin C seem justified? One good example is in the brain. As I frequently point out, the brain is a huge energy and nutrient hog and Vitamin C is no exception. Several studies have shown Vitamin C intake correlated with improved cognitive function  and neuroprotection.  But it should be noted that there are other studies that show no correlation between Vitamin C intake and cognitive function. Researchers examining the Rotterdam Study, for example, concluded that "there was no association between cognitive function and intake of vitamins C and E".  Again, though, the majority of studies show some advantage to Vitamin C supplementation.
3) Blood Flow, Nitric Oxide and Erections. There is good evidence that in men struggling with endothelial and erectile dysfunction that Vitamin C can significantly help their situation. I show all the latest evidence for this in my link on Vitamin C and Erectile Dysfunction.
4) Weight Loss. Researchers found that "Vitamin C status is inversely related to body mass. Individuals with adequate vitamin C status oxidize 30% more fat during a moderate exercise bout than individuals with low vitamin C status; thus, vitamin C-depleted individuals may be more resistant to fat mass loss".  But remember this is only for Vitamin C deficient individuals and does not justify megadosing. You can get plenty of Vitamin C from food to overcome these kind of deficits.
5) Cortisol and Stress Reduction. Vitamin C has been shown in a number studies to reduce cortisol during times of stress. For example, one study of ultramarathoners  showed decreases in cortisol from those given megadoses of Vitamin C. Another (almost humorous) study induced stress by forcing subjects into public speaking and mathematical test-taking situations and found that 3000 mg reduced cortisol levels significantly.  More recent research found that 1,000 mg of Vitamin C reduced cortisol levels in weight lifters for 24 hours after lifting.  Animal studies have shown the same thing on animals under stress. 
6) Skin. Vitamin C protects your collagen, a key component to any anti-aging regimen for your Skin and Appearance. Now I know of no study that shows that oral Vitamin C supplementation reduces wrinkling and sagging. However, it is very likely that Vitamin C does reduce or at least protect your skin from wrinkling on a small scale. In fact, researchers just found that Vitamin C protects skin cell DNA through fibroblast stimulation  and think it may help to actually heal the skin. NOTE: Vitamin C definitely can reduce wrinkling if applied topically. See this link on Skin and Topical Vitamin C for more details.
7) Immunity. Vitamin C has many immune-boosting properties but hasn't done quite as well in the studies as one might hope. However, one thing that most experts agree upon, as I document in my link on Immunity, is that Vitamin C often lowers the effects of colds and flus. The worst thing about colds and flus is that they last for a week or two and make you miserable with drainage, runny nose and similar symptoms. Vitamin C seems to help significantly with these type of symptoms, thus putting it somewhat in the "pain reliever" category, i.e. making your cold or flu more tolerable while you give your body's own immune system time to recover.
8) Mood. The above study, and others as well, have found that Vitamin C often improves mood and reduces depression. 
9) H. Pylori. A large percentage of people are infected with H. Pylori, which leads often to gastritis, stomach cancer and probably a few other problems that we have not discovered yet. Vitamin C seems to make life miserable for the little critters and one study of patients so infected found complete elimination of the H. Pylori in almost a third of the patients.  NOTE: These patients were given 5 grams daily.
10) Lead. Megadoses of Vitamin C have substantially reduced baseline lead levels in several studies. For example, one study on smokers showed that a gram daily of vitamin C reduced lead levels by a whopping 80%. 
11) Libido. One reader reported to me - this is anecdotal of course - that Vitamin C helps significantly with his ADHD and libido. I explain why this may be the case for some men in this link on Vitamin C and Libido.
12) Inflammation. There is gathering evidence that Vitamin C may improve inflammation, particularly in at risk men. One prominent example was a 2009 study showed that CRP (C-Reactive Protein) can be reduced by 25% in men (and women) with levels > 1.0 mg/L.  What is remarkable is that this was achieved with a mere gram per day, which is a fairly low intake by "Linus Pauling" standards.
13) Homocysteine Protection. Hopefully, you have read my Erectile Dysfunction and Homocysteine page. Basically, that article warns men that even mid normal homocysteine levels can lower nitric oxide and increase the risk of erectile dysfunction (and heart disease). Vitamin C may actually be a huge help in this battle because it protects against oxidized LDL, which is what can increase atherosclerotic plaque. 
My strategy is to take 3 500 mg doses of Ester-C or one of the equivalent supplements. And I will take more if I feel as if I might be getting sick. And I always take Vitamin C with no fat in my stomach. Of course, this is no problem for me as I almost always consume a Low Fat Diet anyway.
NOTE: Regular Vitamin C is acidic and can be hard on the stomach: you may want to consider the Ester-C (500 mg) , which is neutralized, if you do decide to take it.
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4) Eur J of Clin Nutr, 2003, 57(Suppl1):S54ï¿½S57
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6) Am J Epidemiol, 1996, 144:275ï¿½80
10) Int J Sports Med, 2001 Oct, 22(7):537-43, Peters EM, Anderson R, Nieman DC, Fickl H, Jogessar V., "Vitamin C supplementation attenuates the increases in circulating cortisol, adrenaline and anti-inflammatory polypeptides following ultramarathon running."
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14) Free Radic Biol Med, 2009 Jan 1, 46(1):70-7, "Vitamin C treatment reduces elevated C-reactive protein"
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