I’m not sure if the shots I had used were inject-able glucosamine, but the ones I had tried years ago with my gelding lasted about 6 months on one shot application; they were through the vet. I think since then there have been others released that are applied by the owner, so maybe that is what you’re talking about? I know when I did whatever shots the vet recommended, he also recommended an oral supplement to support the shots to last a little longer. I did see a change in personality the day after the shots, so they did work. They just cost me a pretty penny and could only promise 3-6 months of relief for him.
Since the shots were quite expensive and my gelding has diagnosed arthritis in both hocks with dwindling use for riding or showing, I stick to oral supplements now. I’ve had the best results in oral supplements by sticking to ones with the NASC seal, which means that they have been tested to contain what they claim to contain and at the levels stated.
I used to recommend Cosequin, but they mysteriously lost the NASC seal and upped what their recommended dose should be, then it mysteriously stopped working for my gelding. So I think there was a formula change. I then switched to a bunch of different things, eventually landing on Legacy Pellets for Senior horses. Unfortunately the barn owner didn’t give him the right dosage, so up until I switched to SmartPaks and smart supplements (cheaper than what Legacy was per day) I’m not entirely sure if it worked or if the failure was due to poor dosing.