Nothing here to help in what you have now, but it's worth looking at your posture on the toilet. Sitting forward and with your heels lifted up appears to be a better way to do it. In fact, put a thick book under your feet to help get the right posture. There is some medical thought that the traditional way of squatting is best of all. It opens up the passage from the large colon to the anus so the movement is unimpeded.

Rehydration is another issue for bike riders. I find on randonnees that if my rehydration is not adequate, and my intake of things such as energy and granola bars has an influence on how my rear end feels. I know that on one 1200, my efforts were like trying to pass big bars of concrete, and the horroids were next on the scene.

I've never sought medical intervention, and the few I have had have disappeared within a fortnight.

So ultimately, my recommendation for prevention is to ensure you remain adequately rehydrated, and take in even more if you are eating dry energy foods.

Footnote: My ex father-in-law had horroids some years ago and had the injections. These are fraught with danger as he found out -- the doctor missed the target and my FIL ended up quite ill with a serious infection.