I casually added a few things to my Viagra last night...but experienced a significant improvement in response! I'm trying to figure out what it was, and I think it was the grape seed extract that did the trick.
I haven't read much about grape seed extract but according to this well-researched article (LINK HERE), from the folks at Life Extension (who seem to generally know what they are talking about), grape seed extract can increase testosterone levels by blocking the enzyme (aromatase) that converts testosterone to estrogen. (the rest of the article is worth a look, by the way, since it covers a lot of good research on sexual health herbs).
As far as I can tell, there are really no negative effects to taking grape seed extract. However, if less testosterone is converted to estrogen, this could raise testosterone levels. While this may seem like a good thing, the body might respond to this higher level by making less of it. So, my guess is that it's probably ok to try it but not every day.