The Principles of Formula Writing
By Dr. Xiu Ling Ma

What is meant by "principles" of writing a formula is mainly the theory of compatibility between the King (principle herb), minister, assistant, and guiding herbs in the herbal formula.
King herb (principle)
A king herb is one which produces the primary effects in treating the cause or the main symptoms of a disease. It dominates the whole formula.
Minster herb
A minister herb has the primary function of helping to strengthen the effect of the king herb and secondarily to treat symptoms that accompany the disease that the king herb is treating.
Assistant herb
The duty of the assistant herb can be divided into three categories.
1. Assist the king and minister to strengthen their therapeutic effects or treat less important symptoms by its own action.
2. Reduce or clear away the toxicity of the king and/or minister herbs so as to prevent the appearance of toxic effects or undesired side effects.
3. Reduce the potential for vomiting in serious cases due to the potent effects of the other herbs. The assistant herb performs this function by opposing the irritating property of the king herb without lowering its therapeutic effects.
Guiding herb
Guiding herbs can be subdivided into two types:
1. Known as the "medicinal guide" which leads the other herbs in the prescription to the affected site in the body.
2. Known as "mediating herbs", these herbs coordinate the effects of the other herbs in the prescription.
The establishment of the king (principle), minister, assistant and guiding herbs in a prescription should be based on their respective importance of effect, potency, and dosage.
The dosage of an ingredient in a formula is not the only basis for the determination of the king herb. Its usual dosage should be taken as a reference. Metals, stones, shells, and other heavy herbs are not necessarily the king herbs even though they appear to dominate in weight dosage. On the other hand, some poisonous herbs, used in small dosage may be the king herbs rather than minister, assistant or guiding herbs.