It shouldn't be, but I can't say for sure. Some supps like Saw Palmetto work against the pro-sexual effects.
I'd need more information. How do you react to Viagra? Do you need it often? At what dosage? I find if you don't need Viagra it doesn't do much. (And incidentally, it won't do anything for psychologically induced impotence.) The xantho and cndium work in the same way and gives me a little "extra" ridgidity but for some guys who are shut down it's a life saver. The only difference is in the duration of the effects. Viagra lasts longer.
So I guess what I'm saying is, you either don't need it or you have that's more severe problem than it can cure. (Sorry, bro, ain't trying to embassass you. : ) )
Try adding 4 grams of arginine with it. That should help. And if you can tolerate it, yohmbine increases blood flow to the extremities, but personally, I hate that stuff. (Makes be as jittery as a 16 year old boy in a monestary).