Like others, I give my dogs glucosamine and it seems to have clear health benefits, but I have tried numerous dosages for myself and when I consume any meaningful amount it gives me absolutely wretched headaches about a day or two after consumption. Something to be aware if you start taking glucosamine and get headaches, it’s probably the glucosamine.
Perhaps you have some undiagnosed shellfish allergy? FWIW, I've noticed iHerb has started selling shellfish-free glucosamine.
I had that thought but have never formally tested it. I do eat an above-average amount of crab, clams, scallops, shrimp (tail on), etc... and I always feel great afterwards. Obviously a glucosamine tablet is far more concentrated, so I could very well be allergic.
I’ll definitely checkout that brand though!