They study the best way to take medications and give guidelines. I probably would not go against that. The whole process of dissolving and injecting with syringe into the anus is not easy. I can't imagine that would be preferable to just waiting a bit longer or giving a bit of prediction to when you may become intimate and planning for it. That's just my opinion. I would not go against the way they have designed the drug to work in this semi extreme way.
Could be. It gets there faster, but thus stops working more quickly. Viagra has a ton of possible side effects, and they will be potentially worse doing it this way. Since you're not a medical professional, your syringe might not be sterile, which can cause problems. If you're on other meds that might interact with Viagra, that will be enhanced. Remember, the original purpose of this substance was to relax blood vessels, and if you overdo it, you can have consequences with the heart and other parts of the body the drug was originally intended to affect. By altering the way the drug was manufactured to be used, again, you are on your own, it wasn't studied to be used this way, it was made to be ingested. You will still eventually digest it and filter it through the liver, but differently than if you ingest it. This doesn't mean you will get a bad result, and maybe you're experienced in doing this and so you could have no problems, but yeah, it can be negative, and some of the negative things with these kinds of meds can be pretty unwelcome.