Because sometime i overspray and its hard to get erect if oversprayed and feels effective most of the time but sometimes does let me down
It sounds like you are thinking of routinely using Viagra just in case you happen to overspray. Wouldn't it be better to learn how not to overspray in the first place?
Like any drug, if used regularly, your body will eventually become accustomed to it, and its effectiveness will lessen. There are threads on here of guys over time needing to take steadily increasing doses - I can recall one guy asking if it was safe to up his dose to 200mg a pop! (NO!!) If you start taking it when you don't need to, you run the risk that it will be ineffective when you get to an age when you do need it.
As others have noted, while Viagra may help your erection, it may cause problems with you cumming. This may sound good, but can also be extremely frustrating for both you and your partner if you just can't cross the finish line. There are a whole gammut of other side-effects that Viagra carries - any reputable supplier will list them. The most common in my experience is hot flushes/blushing - many times an SP has asked if I am alright, as I have gone very red. Other symptoms that I have occasionally experienced have been diarrhea, and migraine-type symptoms (thumping headaches, blurred vision, etc.). Only happen very occasionally, not every time and not all together, thank goodness - but it pays to be prepared.