Nifedipine increases blood flow, by relaxing the walls of the blood vessels, allowing more blood to flow. It helps fissures because it increases blood flow to an area which tends to have low blood supply, and because it also relaxes anal muscle, reducing spasms.
I just looked up diosmin, and it looks as though it does the opposite - constricts veins, increases venous tone. Which may be what you want, to cure hemorrhoids, I don't know.
I have been wondering if continuing to use nitroglycerine (has the same effect as nifedipine) might be aggravating my hemorrhoid, I honestly don't know enough about hems to be sure. I never had any before this!
Edit - apparently "some Preparation H products have
phenylephrine, a drug that constricts blood vessels", so it looks as though fissure drugs and hemmie drugs might actually be working against each other. I might have to ask my CRS about that.