What the studies actually examined
Glucosamine is among the most studied substances in this field, yet the studies are not comparable with one another. They differ in the form used, the daily dose, the duration, the baseline condition of participants and the funding.
A substantial share of the better-known work used crystalline glucosamine sulphate at a fixed daily dose, in countries where it is licensed as a medicine. That is not the same as an over-the-counter combination product.
Why the results diverge
Endpoints are largely subjective: pain and function are captured through questionnaires and respond strongly to expectation. Musculoskeletal symptoms also fluctuate on their own, which is easily misread as an effect without a control group.
Large, independently funded trials tended to be more reserved than manufacturer-linked ones. That is a known pattern and a reason to read funding statements alongside results.
The form is part of the result
Sulphate and hydrochloride contain the same molecule but differ in the share of pure glucosamine and in how much research exists. Transferring a sulphate result to a hydrochloride product is an assumption, not a finding.
That is why the form on the label is not cosmetic. Without it you can neither calculate the active amount nor relate the product to the literature.
What may be said in the EU
Health claims are permitted only if they appear on the official authorised list. No claim about joints or cartilage exists there for glucosamine. Such statements in advertising are unlawful, however plausible they sound.
What is permitted is the vitamin C claim on normal collagen formation. That is why reputable packs in this category reference vitamin C and not glucosamine.