It was just a matter of time - an army of guys, most of them young, began using Viagra and the other PDE5 inhibitors for recreational purposes almost from the day they came out. Of course, middle aged guys often need to use these because of low nitric oxide levels, but there is a tidal wave of younger guys using these, not because they need them, but because they want additional confidence, ehancement and insurance if you will. Can you say "Erection Steroids?" That is essentially what Viagra and other similar drugs have become - recreational vehicles for pleasure and assurance.
It's no wonder as males today are under tremendous pressure to perform: Hollywood, advertisements, even porn present guys as needing almost superhuman physical powers. The Guardian recently reported that a fourth of guys feel "completely inadequate in the bedroom".  One study as far as back as 1999 showed 3% of young club attenders had tried Viagra (sildenafil)  and that percentage has been steadily increasing under societal pressure ever since. As early as 2004 Express Scripts found that among its 5 million members, by far the fastest growing segment was the younger male 18 to 45 crowd.  Express Scripts, a PBM that handles prescriptions for insurance companies and other corporations, also noted that scripts for Viagra were frequently declined because the drug was not for medical purposes, i.e. it was likely being used recreationally.
One more recent 2008 study  performed a survey of young (average age = 21 years) medical students and found that 9% had used Viagra, Levitra or Cialis and 46.7% of those had used them more than 3 times. (NOTE: The above study reported that some of the young males used these to get past condom-related erectile dysfunction.) Again, it is very unlikely that a 21 year old has nitric oxide-related erectile dysfunction and so the great majority of these 9% were undoubtedly off label or recreational usages.
Of course, for most guys everything depends on that erection and so there is a lot of pressure anyway: an orgasm yields a mind-blowing high, a quasi-state of euphoria for about 24 hours and is a nearly essential ingredient for many modern, not-so-forgiving females. Couple all of this with the fact that many females have the ignorant belief that "if the guy is not rock hard, he's not excited," you have the recipe for younger guys to experiment recreationally with PDE5 inhibitors.
The irony is that PDE5 inhibitors such as Viagra have been shown not to help with hardness or erectile strength in young men. If you're middle-aged, that's likely another story but one study of young males showed that there was no improvement in rigidity from taking Viagra.  The only thing that was improved in these young guys was a mild improvement in recovery time. That's not quite the whole story however, as Viagra has been shown to help with psychogenic, or psychological erectile dysfunction as well.  And many younger guys feel that they are not using recreationally but rather to get them over fear and other psychological issues.
So all of this leads to the question, "So what?" In other words, is there a problem with young, or even middle aged guys taking these drugs when they really do not have a nitric oxide deficiency? To answer that question we need to observe that these drugs do NOT just inhibit the PDE enzyme associated with nitric oxide release: they also, to one degree or another, affect PDE enzymes in the ear, eye and stomach. The NY Times reported, for example, that about 2.5% of men "deveolp vision problems that include seeing a blue haze, temporary increased brightness, and even temporary vision loss in a few cases".  Furthermore, yet other guys have reported hearing loss, tinnitis and damage to the optic nerve, which occasionally is permanent. (I cover all of this in more detail in my link on The Pros and Cons of Viagra and Other PDE5 Inhibitors.)
So there are signs that this is affecting multiple tissues in the body in a negative way and yet does not generally cause the kind of major damage needed to cause a pronounced side effect. And that leads to an important question: what if someone uses these drugs for decades? Will there be slow, incremental damage that builds up over a lifetime and increases the risk of stomach cancer or visual/hearing loss? We just don't know and probably won't for a long time.
In addition, there may be a rebound or withdrawal effect in young guys. Anecdotally, this is the case and one animal study shows that in young rats PDE5 was sensitized, which is not something that you want.  This has not been verified in humans, but it does make you wonder if there is some truth to the stories told by recreational Viagra users. By the way, this is NOT true with middle aged and beyond males with erectile dysfunction: studies have shown that Viagra can actually restore normal erectile function and the reason is likely oxygenation of the penis.  Middle aged guys with some impotence have more and better nighttime erections with PDE5 inhibitors and this oxygenation protects certain tissues that trap blood within the penis.
I would highly encourage anyone who is using Viagra or one of the other PDE5 inhibitors recreationally to wean themselves off of these. The side effects can be signficant and we do not know of the long term effects. Also, remember that true erectile dysfunction, even in young males, can be a result of an organic issues such as a varicocele or low testosterone. These are serious and need to be medically evaluated.
In addition, do everything you can to get your confidence back through 1) psychological means (one study demonstrated the benefits of cognitive behavioral therapy for example ), 2) Nitric Oxide Boosting Foods, 3) lots of exercise and 4) adequate sleep. Most younger guys can easily recover using these simple techniques.
1) The Guaridan UK, Dec 7 2008, Amelia Hill, "Men lack confidence in boardroom and bedroom"
2) Journal of Sexual Medicine, pub'd online on Mar 4 2008, 5(10):2414-2418, "Recreational Use of PDE5 Inhibitors by Young Healthy Men: Recognizing This Issue Among Medical Students"
3) NY Times, Monday, June 7, 2010, Health Guide, Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc
4) BMJ, 1999 March 6, 318(7184):669, "Sildenafil (Viagra) is used as a recreational drug in England"
6) International Journal of Impotence Research, 2003, 15:225 228, "Sildenafil does not improve sexual function in men without erectile dysfunction but does reduce the postorgasmic refractory time"
7) Molecular Pharmacology, Jul 2005, 68(1):226-232, "Erection Capability Is Potentiated by Long-Term Sildenafil Treatment: Role of Blood Flow-Induced Endothelial Nitric-Oxide Synthase Phosphorylation"
9) Int J Clin Pract, 2000 Nov, 54(9):561-6, "Sildenafil citrate (Viagra) is effective and well tolerated for treating erectile dysfunction of psychogenic or mixed aetiology"
10) J Sex Med, 2007 Jul; 4(4 Pt 2):1117-25, "Integrated sildenafil and cognitive-behavior sex therapy for psychogenic erectile dysfunction: a pilot study"