Pneumonia and influenza are common among travelers returning from China—talk to your doctor about inoculations before you leave. If you need to buy prescription drugs, try to go to the pharmacies of reputable private hospitals like the World Link Medical Center. Do not buy them in street-side pharmacies as the quality control is unreliable. Staff at the hospitals listed here speak English; indeed many doctors are expats.
Most pharmacies carry over-the-counter Western medicines and traditional Chinese medicines. By and large, you need to ask for the generic name of the drug you're looking for, not a brand name. Oral contraceptives are also available without prescription, but quality in regular pharmacies varies.