Combination of Chinese medicinals means to use various medicinals jointly in a formula or prescription for producing the desired therapeutic effect and reducing toxic or side effects.

Typically, one batch of medicinals is prepared as a decoction of about four to eighteen substances. Some of the ingredients are considered as main medicinals, and some as ancillary medicinals. Some ingredients are added in order to cancel out toxicity or side-effects of the main ingredients. On top of that, some medicinals require the use of other substances as catalysts.

TCM practitioners, through their experience, have generalized seven basic relations, or seven compatibilities, among Chinese medicinals as listed in Fig. 16-1.

Mutual reinforcement* (synergism*) means interaction between medicinals in which medicinals of a similar nature reinforce each other's effects if combined. For instance, rhubarb* and mirabilitum* are each purgatives that produce a more reliable and stronger purgative action when used together. Mutual assistance* means medicinal interaction in which two or more medicinals are used in combination, one being the principal substance while the others play a subsidiary role to reinforce the action of the former. For example, huangqi (radix astragati) has the effect of tonifying qi and promoting water flow, which is usually applied in combination with fangji (Radix Stephaniae Tetrandrae) to reinforce the former's action of promoting water flow. Mutual restraint* is another medicinal interaction in which toxicity or side effects of a medicinal are counteracted by another. Fresh ginger and alum* each reduces the toxicity of pinellia* and arisaema*. Pineilia and arisaema are restrained by ginger and alum. In mutual suppression*, a property of a medicinal neutralizes the toxicity of another. Fresh ginger and alum each reduces the toxicity of pinellia and arisaema; therefore, ginger and alum suppress the toxicity or side effect of pinellia and arisaema. Mutual inhibition* is an interaction of two medicinals when they are prescribed in a same formula, in which a property of one medicinal weakens the action of another; for example, raphanus inhibits the action of ginseng. Incompatibility* means the medicinal interaction in which a toxic reaction or side effects will occur if two incompatible medicinals are taken in the same formula. And acting singly means a medicinal can be used alone to treat patient, such as jinseng decoction. Additionally, prohibited combination* is the relation of medicinals whose combined use is prohibited in a prescription.

In the formative period of TCM, some concerns were raised about the intermixing of herbs. This could be considered the genesis of cautions about medicinal interactions. In traditional herbal pharmacies in China there is an ancient recitation of "eighteen incompatibles" or eighteen clashes; according to this theory, some mixture of the eighteen medicinals would cause adverse effects. There is also "nineteen antagonistic medicinals", or nineteen fears, one of the nineteen medicinals may restrain or neutralize the other's action.

The common administration of Chinese medicinals may be oral, external or topical. There are a great many preparation forms. Forms such as decoction, pill (coated or not coated), pellet, tablet, soluble granules, soft extract and wine, are usually prepared for oral use, while forms like ointment, plaster, medicinal strip, lotion, eye drops and suppository are used externally. Honey and paste are often used to mix in with the pills as binders. However, decoction is still the most widely used form at present, and it may be prepared either by the patient or the hospital. A dose of Chinese medicinal decoction is usually taken twice a day and is usually decocted twice, though nourishing ones may be decocted three times. The decocted juice for a dose for an adult is about 250-300 ml. A decoction is usually taken warm.

Clinically, there are many types of formulas (for decoction), including major formula*, minor formula*, slow-acting formula*, quick-acting formula*, and combined formula*. Major formula is a formula or prescription which consists of many ingredients or a large dosage of medicinals, used for treating serious diseases with exuberant pathogens or diseases of the lower energizer. In contrast, a minor formula contains only a few ingredients or small dosage, and is used for treating a mild illness. Slow-acting formula is composed of ingredients that act slowly and is indicated in the long term treatment of chronic cases, while quick-acting formula is employed for immediate effect in the treatment of emergency or critical cases.