Traditional Chinese Medicine (TCM) is one of the great herbal systems of the world, dating back to the 3rd century BC. Yet throughout its history it has continually developed in response to changing clinical conditions, and has been sustained by research into every aspect of usage. This process continues today with the development of modern medical diagnostic techniques and knowledge. Chinese herbal medicines are very safe when prescribed correctly by a properly trained practitioner. Over thousands of years, experienced doctors have compiled detailed information about the pharmacopoeia and placed great emphasis on the protection of the patient. Allergic type reactions are rare, and will cause no lasting damage if treatment is stopped as soon as undesired symptoms appear. The primary difference between Chinese and Western medicine can be described as Chinese treats the Yang and Western treats the Yin. Everything in the universe can be described in terms of Yin or Yang. This is one of the underlying philosophies of Traditional Chinese Medicine. When applied to medicine in general, Western medicine acts upon the Yin of the body, the substance of the body, the actual cells and chemicals. Traditional medicine works more on the energy that animates those cells.