My partner and myself with be travelling and studying in china for 6 months.

She is chronic asthmatic and has serious allergies which means she needs quite a bit of prescription medication. Do we need to have it declaired, can we pack it in our checked luggage and is there a limit as to how much she can take with? We have prescriptions for everything, even paracetamol as she cannot take just any pain medication due to her allergies and asthma. Should I get a letter from our doctor as well just to be safe? First time travellers from South Africa.