House, you would need to ask your doctor about dose adjustments. However, verapamil is known to be an inhibitor of an enzyme called CYP450 3A4, which is what the body uses to remove a wide variety of medications from the body. The enzyme metabolizes the drugs, which breaks them down.
Viagra is one of the medications that is metabolized by 3A4, so by inhibiting 3A4, verapamil would increase serum levels of viagra, and this suggests taking a smaller dose of viagra to compensate for this effect. I don’t know by how much, but in any case the general rule is to take as little as you can and still have a good effect. There’s no real benefit to taking more anyway. So maybe you should experiment, beginning at maybe 25mg, and just see how little you can get away with and still have it work.
But I should also say that verapamil may also inhibit the metabolism of estrogens, which is generally not a good thing for men because it increases estrogen levels and reduces testosterone levels.
I’m curious about the choice of verapamil for blood pressure. It is one of the original CCBs, but there are newer ones that seem to work better, such as Norvasc, without some of the downsides. Do you know why your doctor picked that drug in particular?