I now have a different knee problem, but i used to suffer from osgood schlatters disease.
I found that it (my left knee) would just "go" underneath me, and i couldn't put weight on it for about half an hour.
I can only describe this as feeling like it was jelly, as it used to just give way.
It was stopping me from participating in sport, so i went to see my G.P.
Like you, he prescribed me ibruprofen medicine, but i found that it didn't help at all.
He then tried several creams, but i have found that creams never work.
The best solution that i found was to wear a knee support brace, which sounds ominous but it's really not! They are discreet, and give your knee lots of support. You don't need to go anywhere special to buy these, in fact, i bought mine from a local "Boots" shop!
Apart from this i found that pain relievers like paracetamol, or perhaps something stronger like co-codamol, which you can only get on prescription, were useful.
Hope that helped:-)