Taking Ritalin to China, what letter do I need from my doctor?

I am going to China for two weeks in April and I have a prescription for Ritalin. I understand that I need to bring some sort of form letter from my doctor that states that I need this medication. What exactly does my doctor have to write/explain? I also have a few other pills that I take infrequently, mostly anxiety pills and sleeping aids. Do I need a letter for those too?

I have already contacted the Chinese embassy in my city over this but they did not respond to any emails.