There have been medical studies looking for tachyphylaxis in response to continued use of viagra (the phenomenon you’ve described), but as far as I know, none has concluded that it occurs. This is not to say that it can’t happen or that won’t happen to anyone, but there is no proof that it does occur yet.
However, I can tell you from personal experience that continued use of generic cialis has slightly diminished its effects on me I think. It’s not too severe yet, and the good thing about cialis is that when I take one, I can feel it’s effects for like 4 days or even more sometimes. So basically you can take it less frequently than viagra. I’d try cialis if I was you - it targets more effectively the enzyme that inhibits erections, leading to longer and better results and fewer side effects.