Apart from very few single drugs of all the prescriptions used clinically, the great majority of them are compound drugs consisting of two or more drugs. The reasons are that the action of a single drug is usually limited, and some of them may produce certain side effects or even toxicity. But when several drugs are applied together, ensuring a full play of their advantages and inhibiting the disadvantages, they will display their superiority over a single drug in the treatment of diseases. This can be illustrated in the following three aspects.

Firstly, drugs of similar action, if used simultaneously, can strengthen the therapeutic effect for serious diseases. For example, the synergism of Mangxiao ( Natrii Sulphas ) and Dahuang (Radix et Rhizoma Rhei) can enhance the therapeutic effects of eliminating pathogenic factors by purgation in the treatment of serious heat accumulation syndrome, e.g. Da Chengqi Tang.

Secondly, drugs of different actions in combination can broaden the therapeutic scope in the treatment of complex conditions. For example, Renshen (Radix Ginseng) capable of reinforcing qi and Maimendong (Radix Ophiopogonis ) of nourishing yin in combination has the action of reinforcing both qi and yin for deficiency of both qi and yin, e.g. Shengmai San.

Thirdly, drastic or noxious drugs may be applied with some drugs capable of reducing or removing their side effect or toxicity so that they are not likely to generate or at least produce less damage to the body resistance or toxic reaction. For instance, Gansui (Euphorbia) has the function of eliminating retained fluid, but is drastic and toxic in property. If Dazao (Fructus Ziziphi Jujubae ) are added to it, they can alleviate its side effect, e.g. Shizao Tang.

Hence, it’s obvious that a rational and appropriate compatibility of the drugs contributes to the full play of the drugs efficacy and helps reduce to its maximum or get rid of the toxicity and side effect of drugs. That's why compound prescriptions are so widely used. To achieve the above requirement so that the made prescriptions can meet the clinical syndromes to the greatest extent, it is imperative that the drugs be chosen and prescriptions made with flexibility to suit the specific syndromes under the guidance of the formulation principle.