Funny you should ask! I lived on Robaxacet when I had my fissure. I just now mentioned that to another boardie too. I also thought that a muscle relaxant should help with butt spasms, so I tested it and, for me, it worked. It didn't eliminate the pain, but it definitely took the edge off.
I didn't want to use the anti-anxiety meds my doc gave me too much. My fissure gave me a whopper of an anxiety problem, and the drugs helped, but I didn't want to develop a dependency.
Robaxacet and ibuprofen made a huge difference for me. Elphie also used that combo and it helped her out a lot too. I've recommended that combination a few times on this forum. :D

One thing that has come out of the search for my mystery neuro-psych issues is that I am suffering from both depression and anxiety/PTSD. Due to that, I was already on anti.depressants and anti-anxiety meds before the fissure came to make my life happier.
With my history, I do feel like I am walking a tightrope at times, but that is why my sponsers have told me for years to work a good program. It is times like these when it pays off!
This is when you learn the difference between the kindling and the big wood!

Sterlingbird, if you ever need to talk from both perspectives I am here.