Lets assume a young, 22 year old with full sexual usage of his penis takes a viagra.
What happens? Anything?
Some schoolboys took Viagra last year in the UK:
http://news.bbc.co.uk/1/hi/england/hampshire/dorset/3205747.stm
Taken to hospital but basically okay.
Since it works by increasing blood flow to the penis, I would assume that in someone who already has a healthy blood flow to the penis, he would only be a bit “more so”, but if you’re asking whether his penis would explode, I would doubt it, because as it became more and more taut, eventually painfully, agonizingly so, before it exploded, he would lose his erection.
Unless he was some kind of masochist, in which case all bets would be off.
[goes off snickering at all the Straight Dope penis-owners whose mornings she has just ruined]
The schoolboys weren’t taken to the hospital because their penises exploded–they were taken to the hospital because they had taken DRUGS!!! :eek: and the authorities knew that that’s what you do with junior high school kids who have indisputably taken DRUGS!!! :eek:
It says, “As a precaution, paramedics were called…”
I know that sitcoms aren’t always accurate with details like this, but the question reminded me of a late episode of ‘mad about you’ in which the husband (mid thirties) was convinced by his wacky cousin to take viagra ‘just out of curiosity.’
Of course, the usual comedic implausibilities kept him from actually meeting up with Helen Hunt and giving it a fair tryout until they’d done numerous variations on the basic joke of him rushing around through public places with everyone around being surprised and shocked at his presumably obvious state of excitement. (All shots from the waist up of course.) 
Come to think of it, I think that was the occasion on which Jerry Seinfeld put in an appearance, but I’m not sure.
The 13-year old kid next door got into his dad’s Viagra. He’s okay now, but he got 2nd degree burns on his right hand.
I’m 31 and I was taking Viagra for fertility issues (trying to get my wife pregnant, willing to try anything, why the hell not this too, anyone who has had infertility issues knows exactly what I mean, and I sympathize with you). It makes a guy become more easily sexually stimulated through physical contact and it keeps you stimulated and erect longer than if you are not using it. Basically its like I’m 15 again, where the thing is a bit out of control. Note that just taking the pill doesn’t give you an erection, you do have to stimulate the penis physically to get the erection. But I do get big much quicker than usual. It takes 30 minutes to 1 hour to kick in and lasts about 4 hours. I would imagine that if you were even younger than me, that your stamina would be increase, you would get more erections than usual, and you might sport major wood for long periods of time. The only negative I have noticed with it is that it can make my heart a bit racy sometimes, and I do have a healthy heart and I am taking it under doctor’s orders. I didn’t buy them black market or through a spam e-mail. 
Dave Navarro (36), guitarist for Jane’s Addiction, got some Viagra as a wedding gift from his doctor and had great fun for the first 45 mins but the next 8 hours left him with a headache.
I don’t see why Carmen Electra’s husband would need any help, but eh… 
Viagra is a vasodilator. It works by causing the arteries in your member to dilate (open up), thus increasing the blood flow to Mr. Happy. Viagra is “keyed” to the arteries in the nether regions as opposed to arteries in other areas of the body.
There is, though, some “spill over” into the other arteries. That is why the reports of racy hearts and headaches. The arteries in other parts of your body are opening up to allow more blood to flow as well. This is why the combination of Viagra and a heart medication like nitroglycerin can be deadly. They will BOTH open arteries all over the body causing various and sundry nasty things to happen.