PDE5i's are not meant to prolong ejaculation. That's why they're combining ST-SSRI's w/ PDE5's or recommending them w/ PDE5's.
PDE5's working indirectly to combat PE:
A man who fears that he will lose an erection before ejaculating may well increase the speed of intercourse, thus giving himself PE. The underlying problem in these circumstances is loss of tumescence related to the narrowing of the pudendal artery, in which case a PDE5I may be a more appropriate prescription.
They're not made for PE, otherwise the marketing would be much different on them.
Here is a study though that overviews the mental impact w/ PE and PDE5i's:
http://www.ncbi.nlm.nih.gov/pubmed/16422868
NOTE that it says it helped. The mechanism in which they work in the body does not have a direct impact. That's an indirect result above.
Another thing. Lack of blood flow can cause one to kegel constantly to draw in more blood, thus increasing the likelyhood of ejaculation.
PDE5i's would play a huge role in helping w/ the above.
I'm not going to get banned by pming you a source, so I'm not going to do that. You can research online pharmacy reviews and see who's legit or not.
You can take half of it and have it work/not work or you can take the whole thing and have it work/not work. It depends on the individual.
You're going to want to half it though, just to see how low of a dosage you can get away with. This can also save you a lot of money.
I would look @ cialis over viagra first as it's reported to have less sides.
