Compatibility of Chinese medicinal herbs refers to the combination of more than two herbs with purpose in the light of the clinical requirement and medicinal properties and actions. It is the main method of medicinal application in clinic and also the basis of making up formulae of Chinese medicinal herbs.

During clinical practice, we only use a single to treat a disorder and fulfill its therapeutic purpose if the case condition is simple and light. But on occasions when a disease is accompanied by other diseases, or it is due to invasion of both superficies and interior by pathogenic factors, or asthenic syndrome is complicated with sthenic syndrome, or cold syndrome accompanied by heat syndrome alternatively, a single formula can fail to achieve desired effects. Furthermore, some medicinal herbs used in single form may produce toxic side-effects or may be harmful to a patient. Therefore several Chinese medicinal herbs must be used in combination according to their specific properties so as to extend the circulation of their treatment, decrease their toxic side-effects and gain better therapeutic effects.

Chinese medicinal herbs may have complicated changes by combination. Some may reinforce or decrease their effects, moderate or eliminate their original toxic side-effects, whereas others may produce toxicity and poor reactions. The relationship between a single and the compatible ingredients was generalized previously by ancient physicians as seven aspects, namely, singular application, mutual reinforcement, mutual assistance, mutual restraint, mutual detoxication, mutual inhibition and incompatibility. Except that singular application means using a single medicinal herb, the other six aspects mainly denote the relationship of compatibility between Chinese medicinal herbs.

Mutual reinforcement: That is, two or more ingredients with similar properties and effects are used in combination to reinforce each other''s action. For example, Dahuang (Rhizoma et Radix Rhei) and Mangxiao (Natrii Sulfas) which are both purgative, after they are used in combination, can reinforce each other''s original purgating action; Honghua (Pros Carthami) and Taoren (Semen Persicae) of blood invigorating herbs used in combination can reinforce their action of invigorating the blood and removing blood obstruction.

Mutual assistance: That is, Chinese medicinal herbs that are not certainly similar but have some relationship in the aspect of medicinal properties and actions are used in combination, in which one herb is taken as the dominate factor and the others as its assistants to raise its therapeutic effects. For example, Huangqi (Radix Astragati) with the effect of tonifying qi and promoting the flow of water is used in combination with Fangji (Radix Stephaniae Tetrandrae) with the effects of promoting the flow of water and permeating the dampness, the latter reinforcing the former''s action of promoting the flow of water, so their combination can be used for edema due to spleen-deficiency; Shigao ( Gypsum Fibrosum) with the effect of clearing away heat and purging fire in combination with Xixin (Herba Asari) can purge fire to relieve pain and treat toothache due to stomach-fire.

Mutual restraint：That is, mutual restraining effect of different medicines to weaken or neutralize each other''s harmfulness, such as toxicity or side-effects. For example, the poisonous action of Banxia (Rhizoma Pineliae) or Nanxing (Rhizoma Arisaematis) may be decreased or eliminated by Shengjiang (Rhizoma Zingiberis Recens), therefore we say there is mutual restraint between Banxia (Rhizoma Pineliae) or Nanxing (Rhizoma Arisaematis) and Shengjiang (Rhizoma Zingiberis Recens).

Mutual detoxication： That is, one medicinal herb can relieve or remove toxic properties and side-effects of the other. For instance, Shengjiang (Rhizoma Zingiberis Recens) can be used to relieve or eliminate the toxicity or side effects of Banxia (Rhizoma Pineliae), Nanxing (Rhizoma Arisaematis), etc., therefore it is said that Shengjiang ( Rhizoma Zingiberis Recens ) can detoxicate the toxicity or eliminate the side effects of these medicinal herbs. From the mentioned above, we can see that mutual restraint and mutual detoxication actually refer to same thing, yet each one is putting its constraint into the others.

Mutual inhibition: That is, when two herbs are used together, one herb and the other act on each other, resulting in their original actions being weakened, even lose of their medicinal effects. For instance, the qi-tonifying effect of Renshen (Radix Ginseng) can be weakened by Laifuzi (Semen Raphani). So we say there is mutual inhibition between Renshen (Radix Ginseng) and Laifuzi (Semen Raphani).

Incompatibility: That is, toxic reaction or side-effects may result when two incompatible ingredients are used in combination. For instance, there are eighteen incompatible medicaments which are believed to give rise to serious side effects if given in combination.

In clinical application of medicinal herbs, we should make the widest possible use of the two kinds of ingredients with the relation of mutual reinforcement or mutual assistance so as to make full use of their coordination and rein-forcing each other''s action and to raise their therapeutic effects and extend the range of their treatment. When we use medicinal herbs with toxicity or severe side-effects, we should choose the herbs with the relation of mutual restraint or mutual detoxication with the purpose of weakening or eliminating each other''s toxic action or side-effects. The medicinal herbs with the relation of mutual inhibition and incompatibility should be avoided as much as possible to use in combination so as to prevent the therapeutic effects from decreasing or losing, or to stop producing toxin and side effects.