Hotels usually have access to a doctor if you need one. If your condition is serious, you will be taken to hospital. In emergency, you may call: 
SOS First Aid Centre
Qianmen West Street
Tel: 66014433 /120

There are also two private clinics offering Western qualified doctors, medicines and treatment. Both run a 24-hour service and doctors and nurses can speak English. These clinics are not cheap, but they will accept payment by credit card.
Asia Emergency Assistance (AEA)
International 14 Liangmahe
South Road 1/F Chaoyang District
Tel: 64629100

International Medical Centre (IMC)
Lufthansa Centre (Ground Floor)
Room 5106 50 Liangmaqiao Road
Tel: 64651561