Before we talk about how to handle this touchy subject, let me first tell you what I know about Viagra. It is my understanding that this drug isn't capable of 'giving' a man erections. In other words, Viagra cannot raise the dead.
A man must be with a partner that arouses him, otherwise Viagra would be as useful as a placebo (sugar water) pill. Knowing this, you can certainly make use of this information in order to protect and even flatter your sexual partner's egos.
The woman who dropped you because she thought your erections were due to a drug instead of her sexiness, really needed help. If her sense of attractiveness is verified by a man's erection, she's in big trouble. This is her problem, not yours. A more intact woman wouldn't react the way she did.
The larger question here is, do you need to tell that you are on this drug, and, if so, at what point do you tell? If you are having casual sex, then you really don't need to feel obligated to tell your entire story. However, if you are in a relationship with someone, then your medical condition would naturally come out.
The point isn't, to tell or not to tell, but rather that you need to be careful about who you allow yourself to become emotionally involved with. You need to be sure that the woman you choose has a strong sense of self before you become involved. Once involved, your own revelations will come naturally, and they will be received without consequence.
Bottom line, choose your partner wisely. In fact, use my test, Ready for Love? to help you assess whether a prospective partner is a safe bet.