Viagra. Cialis. Levitra. These are a miracle in a bottle, right? Well, not for a huge percentage - around a third - of middle-aged and beyond men. These drugs fail and fail miserably for them - they simply do not work. (And sometimes failure occurs with these PDE5 Inhibitors due to nasty side effects including headaches and hearing, vision and stomach disturbances.)
I have already documented many reasons for their lack of effectiveness in my link on Why Viagra Doesn't Work. However, I want to cover another important reason for Viagra failure here: lack of baseline nitric oxide production in men that are 40+.
This point was emphasized in a Recent Interview with Nathan Bryan, one of the world's foremost experts on nitric oxide. In this interview, Dr. Bryan explained how PDE5 Inhibitors work, which is by inhibiting the activity of an enzyme that breaks down nitric oxide. In other words, and as I have explained often on this site, Viagra and other similar drugs do not actually directly increase NO, they merely allow it to stay in your system longer. This is only effective if you have sufficient baseline nitric oxide production in the first place. As Dr. Bryan put it, "Without production of NO, this pathway does not and cannot work. In fact, this is why Viagra and Cialis do not work in 100% of the population."
And Dr. Bryan himself has pointed out that most 40-year-old men will only produce half of the nitric oxide that they did when they were 20.  This is a drastic reduction! It's a wonder us old timers can even have an erection, eh?
Now imagine what happens when a man has "endothelial dysfunction." His arteries and veins have been damaged, inflamed and coated with extra plaque after years of lifestyle abuse. His nitric oxide is will be in even worse shape. The reduction in blood flow will affect almost every system in his body, including those affect his sex life. And for him taking Viagra, Cialis or Levitra is doomed to failure.
So is there any hope? Are there any natural ways to boost nitric oxide for a man with nitric oxide insufficiency?
Yes! I hope that you've already found many of these ways as you've searched around my site, but let me summarize some of the key ways that you can get your nitric oxide levels back in shape:
1. Diet. One of the ways you can increase baseline nitric oxide production is through consumption of nitrates. I discuss many of these in my book on book Peak Erectile Strength Diet, but spinach and beetroot juice are classic examples. Nitrates are converted to nitrites by bacteria on your tongue and to NO in your stomach and gut. In addition, many of these foods will also enhance and protect your nitric oxide, thus effectively raising nitric oxide levels farther downstream.
CAUTION: Avoid amines when consuming nitrates in the diet. Certain cheeses and meats may combine with nitrates and create carcinogenic compounds called nitrosamines. In addition, avoid consuming supplemental Vitamin C with any fat in the stomach for a similar reason.
2. Neo40. This new supplement works via not allowing supplying baseline nitrates, but also by adding an enzyme that helps the body convert the nitrates more easily to nitric oxide. 
NOTE: You can boost the power of all of these through exercise. Exercise is a potent booster of nitric oxide and researchers believe it works primarily by increasing the bioavailability of the same enzyme that Viagra works on. So then does it really increase baseline nitric oxide production? Well, exercise gets you to the same point through a different route: it increases the number of new capillaries and veins throughout the body, a fact I cover in my link on Exercise and Erectile Dysfunction. This has a similar net effect as #1 and #2 above - more blood flow to more tissues. See my link on Sexercise for a dozen other reasons to exercise as well.