My horse has a spavin. He survives on the optimum rate of Extraflex HA. I believe it has made a very significant difference to his movement. Have tried him on other Condroitin/Glucosamine/HA supplements but this is the best one for him to date.
I used the zerobute for 1 month, IMO it didn't make a palpable difference to him.
I have not tried Zerobute but previusly used No Bute. I then had to change as my local tack shop closed and I started using Bute Free by Gold label. It contains Devils Claw, MSM and Yucca and it was noticeably more effective than No Bute, so might be worth a try if you find Zerobute not very effective.