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