In the compatibility of traditional Chinese medicine, some drugs should be avoided. Shennong's Classic of Materia Medica calls the relationship between these drugs "mutual evil" and "opposition". Later generations have a knowledge and development of compatibility taboos, but there are different opinions in ancient books. The Jin and Yuan Dynasties were summarized as "Nineteen Fears" and "Eighteen Oppositions", which are still widely recognized and applied.

Nineteen fears: sulfur is afraid of mirabilite, mercury is afraid of arsenic, stellera chamaejasme is afraid of lithargyrum, croton is afraid of Petunia, clove is afraid of turmeric, aconite kusnezoff monkshood is afraid of rhinoceros horn, tooth nitrate is afraid of trigonous, official laurel is afraid of lithopone, and ginseng is afraid of trogopterori.

Eighteen opposites: licorice, spurge, seaweed and Daphne genkwa; Bulbus Fritillariae Cirrhosae, Fructus Trichosanthis, Rhizoma Pinelliae, Radix Ampelopsis and Rhizoma Bletillae; Veratrum nigrum, Radix Adenophorae, Radix Salviae Miltiorrhizae, Radix Scrophulariae, Asari and Radix Paeoniae.

Many drugs listed in eighteen evils and nineteen fears are also commonly used to treat gastrointestinal diseases, such as licorice, Fritillaria, Pinellia ternata, Bletilla striata, ginseng, Adenophora, Salvia Miltiorrhiza, Scrophularia, Paeonia lactiflora, clove, Radix Curcumae, Rhizoma Sparganii, Cinnamomum cassia, tallow, ginseng, Oletum Trogopterori and so on. Therefore, we must pay attention to the application, and we must not use it illegally to avoid adverse consequences.