DMSO is dimethyl sulfoxide. It was a biggie back in the 80s for arthritis pain relief. It is used as a prescription drug for treating bladder ulcers - ouch - I shudder to think how it's administered.
Used off-label for arthritis, there does not seem to be any scientific evidence for efficacy, only anecdotal evidence.
A friend had great results back then; I berated him, as the stuff he was buying was technical grade purchased from a farm supply. He countered that it was 95% pure - in medicine and scientific work, that is absurd, as you don't know what the other 5% might be. Pharmaceutical grade is 99.995%. For reference, in one of our labs, we purchased water for $80 per liter, but it was ultra, ultra, ultra pure and came with a certificate of analysis.
Due to ongoing arthritis in my wrists, I recently ordered some in a roll-on bottle. I tried it and found no effect after 3 days' use. More important, I had stinging, irritation and redness where it was applied. As I write this, I rolled some on my wrist and within 1 minute, the stinging appeared. No more; I'll stick with Voltaren; not perfect, but it does not add skin problems. Gotta go wash this stuff OFF !