Viagra will block an enzyme (I'm too lazy to look it up now as to what its name is) that helps in the reuptake and disposal of nitric oxide in the system.
This enzyme also has something to do with how they eye sees blue.
As a result, some people are highly succeptible (sp?) to having odd blue visual hallucinations of sorts. Those that are less succeptible (sp?) to it are increasingly more likely to get it as the dosage goes up.
The two new(ish) alternatives to Viagra don't have this issues since they manipulate the nitroc oxide levels through a different mechanism, leaving the enzyme levels at appropriate concentrations, and therefore no blue viusual oddities.
There is also a chance of priapism, but that is much more of a risk in the usage of trazadone or the drugs that are injected right into the smooth muscle walls of the erectile muscles.
There is also the flushing of the skin, most pronnounced in the face due to the vascular systems reaction to the higher nitric oxide levels.
For those with heart issues or high blood pressure, this isn't a good thing.