I'd (a) give it a few days to kick in as Rob says. Then if it's of no use I'd (b) ask to speak to another Doctor for a 2nd opinion and explain the situation again.
Also, I'd go to your local pharmacist and explain what's happened, they might have something that you could take over the counter thats just as strong!
I used to regularly have to take 1200mg of Ibuprofen for an ear infection while I was on holiday, it was like having a large peach stone in my ear (well...thats what it felt like) as the swelling was so bad. Luckily some anti-biotics when I got back cleared it
My wife takes Tramadol 2x 50mg for general pain in her hip (she has walking difficulty) and that seems to do the trick (but many GPs are very wary about prescribing drugs that strong!)
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You should also note that it takes approximately 1hour for most tablets to be digested by the stomach...regardless of what they say on the box/bottle. Its just the way the body works.