Herbal medicine can not only produce actions of treating and preventing disorders but also have the harmful effect to human beings. When learning medicinal properties, we should not only know their therapeutic effects but also grasp their harmful actions produced after taking. For instance, cool or cold Chinese medicinal herbs having the effect on clearing away heat may be likely to damage yang; warm or hot ones having the effect on dispersing pathogenic cold may damage yin; drastic ones having the effect on removing pathogenic factors may damage healthy qi；tonics having the effect on invigorating healthy qi may linger the pathogenic factors; ones with lifting yang used in the patient with exuberance of yang may produce the side-effect of lifting yang seriously; ones with the action of lowering the adverse-rising qi can make it worse for the patient with qi-collapse, etc.. For example, Huanglian (Rhizoma Coptidis) cold in nature and bitter in flavor is indicated for diarrhea due to damp-heat, but it is not suitable for diarrhea due to spleen-yang deficiency. Ganjiang (Rhizoma Zingiberis) hot in nature and pungent in flavor is indicated for cough due to lung-cold, but contraindicated for dry cough due to lung-heat. In a word, the preponderant properties of these medicinal herbs which have an unfavorable aspect on human body should be corrected or avoided, which is essential to understand the contraindications of medicinal herbs.

The contraindications are divided into caution and abstention from some disorders according to their degree. Some herbs are not suitable for some cases as for their medicinal natures, but when they are processed, combined with other or given improvement of their administration, they can also be used.

Besides, the contraindication in pregnant women and in compatibility should be known well.

Prescription incompatibility

Prescription incompatibility refers to that some medicinal herbs can not be used together in a prescription, otherwise the toxic effect will be produced harming the patient's health, and even his life. Incompatibility also denotes incompatible medicinal herbs, especially denotes "the 18 incompatible medicaments", and "19 medicaments of mutual antagonisms".

In the 18 incompatible medicaments the following herbs are believed to be incompatible in their actions if given in combination: Wutou (Radix Aconiti) being incompatible with Banxia ( Rhizoma Pinelliae ), Gualou ( Fructus Trichosanthis ), Beimu ( Bulbus Fritillariae), Bailian (Radix Ampelopsis ) and Baiji ( Rhizoma Bletinae ); Gancao (Radix Glycyrrhizae ) incompatible with Haizao ( Sargassum ), Daji ( Radix Euphorbiae Pekinensis), Yuanhua ( Flos Genkwa) and Gansui (Radix Euphorbiae Kansui ); Lilu ( Rhizoma et Radix Veratri ) incompatible with Renshen (Radix Ginseng), Shashen (Radix Aclenophorae Strictae ) , Danshen (Radix Salviae Miltiorrhizae ), Xuanshen ( Radix Scrophulariae ), Kushen ( Radix Sophorae Flavescentis ), Xixin ( Herba Asari) and Shaoyao (Radix Paeoniae). Nineteen medic-aments of mutual antagonisms include Liuhuang (Sulfur) being antagonistic Qianniuzi ( Semen Pharbitidis ), Dingxiang (Flos Caryorphylli ) antagonistic to Yujin (Radix Curcumae ) , Chuanwu (Radix Aconiti ) and Caowu ( Radix Aconiti Kusnezoffii) to Xijiao ( Cornu Rhinocerotis Asiatici), Yaxiao ( Crystallized Mirabilite ) to Sanleng ( Rhizoma Sparganii ) and Guangui ( Cortex Cinnamomi ) to Wulingzhi (Faezes Trogopterorum ). It must be denoted that "being antagonistic" here means "being loath" or "being dislike", which is different from "Mutual Restraint" in the compatibility of Chinese medicinal herbs.

As to the 18 incompatible medicaments and 19 medicaments of mutual antagonisms, they are regarded as ingredients which are incompatible, but some of them were still used in combination by some physician in various dynasties. The conclusion of the 18 incompatible medicaments and 19 medicaments of antagonisms got in modern experiments and research is not completely similar. Therefore, the conclusion has not been confirmed and further research will be made. So we should use them cautiously and generally we should avoid using them in combination.

Contraindication of Chinese medicinal herbs in pregnancy

Some medicinal herbs should be regarded as those that are contraindicated or used with cautions in pregnant women, otherwise, side effects of damaging fetus, inducing abortion or miscarriage may be brought about. They may be divided into two kinds, one being contraindicated in pregnancy, which involve poisonous medicinal herbs and drastic purgatives; the other being used with caution, which mainly includes those for eliminating blood stasis and promoting circulation of qi, the purgatives and part of herbs warming the interior. If this kind of medicinal herbs are used improperly, the fetus may be damaged or abortion (miscarriage) may be induced.

Chinese medicinal herbs contraindicated are Shuiyin (Hydrargyrum), Pishuang (Arsenicum), Xionghuang ( Realgar ) , Qingfen ( Calomelas ), Banmao (Radix Sacchari Arundinacei), Wugong (Scolopendra), Maqianzi ( Semen Strychni ), Chansu ( Venenum Bufonis ), Chuanwu (Radix Aconiti), Caowu (Radix Aconiti Kusnezoffii), Lilu ( Radix et Rhizoma Veratri ), Danfan ( Chaleanthium ), Guadi ( Pedicellus Melo Fructus ), Badou (Fructus Crotonis), Gansui ( Radix Euphorbiae Kansui ) , Daji (Radix Euphorbiae Pekinensis ) ,Yuanhua ( Flos Genkwa ),  Qianniuzi ( Semen Pharbitidis ), Shanglu ( Radix Phytolaccae ), Shexiang ( Moschus ), Ganqi ( Resina Rhois Praeparata ), Shuizhi ( Hirudo ), Mengchong ( Tabanus ), Sanleng ( Rhizoma Sparganii), and Ezhu (Rhizoma Curcumae), etc..

Chinese medicinal herbs used with caution are Niuxi (Radix Achyranthis Bidentatae ), Chuanxiong ( Rhizoma Chuanxiong), Honghua (Flos Carthami), Taoren ( Semen Persieae ) , Jianghuang (Rhizama Curcumae Longae ) , Mudanpi ( Cortex Montan Radicis ), Zhishi (Fructus Aurantii Immaturus), Zhike ( Fructus Aurantii ), Dahuang ( Radix et Rhizoma Rhei )