Glucosamine sulphate is pointless.
Save money and ditch it.
The only time glucosamine has been shown to have anything other than no effect is when it's taken with chondritin or whatever it's called. And even then, the results are inconsistent (I.e. some studies show no effect) or not statistically significant.
So it is most likely just the placebo effect, I'm afraid.
Doctors will recommend it for the same reason they recommend cough syrup to people with sore throats: the sense of ownership and being able to do something about an illness/affliction makes people feel better about it and more positive, despite doctors acknowledging cough syrup, etc. doing bum all.
Just start squatting with correct depth and form and watch your knee pain disappear.