Viagra is a prescription drug, meaning that you have to go to your doctor to get it. If you express your concern with him/her, they will be honest with you as they wouldn't want to prescribe something harmful to you.
Viagra was a medication made by accident, it was for blood pressure. The "side effect" as it were, was erection. They really can't pass it off as a blood pressure medication if someone is walking around with a massive erection. So if you're already taking a blood pressure medication you may not be a candidate. Also in the same realm, they need to check if you show any signs of glaucoma, which effects your eye vessels. There's probably more but that's what I know.
Yet, my best advice is when it comes to your health, don't leave it up to a forum online. We might have some good insights but you never know where the information is coming from if it cannot be verified.
Happy hunting!