I use both baclofen and diazepam. I have good tolerance for both, and have experienced good results with no problems - except that they are becoming less effective. I don’t honestly think this is a tolerance issue (Baclofen, in particular, has low reported issues with tolerance - i.e. needing to keep increasing the dose for the same effect). So unfortunately, I think it’s a matter of the symptoms getting slowly worse, and not the drugs are getting less effective.
I tried gabapentin (in addition to the above), with no effects either positive or negative. I started on a very low dose, which my GP considered had been too low to work.
So under her supervision, I tried again with a higher dose (three times as much). After a month of that, I still noticed no improvement.
In fact, the crunch came when I left the house in a hurry one day, and forgot all about it. Hadn’t popped any in my handbag either.
I expected dire things to happen, or at the very least symptoms to get noticeably worse - to the extent I warned a sensible friend I’d forgotten some important pills I wasn’t supposed to stop suddenly, and told her what they were, in case of trouble.
Despite my fears, I noticed no difference at all from my failure to take them, and that was the final proof for me that they weren’t working. After this incident, I weaned off the proper way.
However, everyone is different. I appear to be someone who is unresponsive to gabapentin, but you may have better luck.
Like Moyna, I was under the impression it wasn’t primarily for stiffness (spasticity) though. I didn’t notice any help with that - or with pain either.