Toxicity of Herbal Drugs
For several reasons it is not possible to establish absolute safety standards for her-
bal preparations based solely on epidemiological studies. First, these types of stud-
ies would be costly. Second, there is little published data in countries where the
major use of medicinal plants occurs and thus general standards based on a limit-
ed number of reports would have little meaning. Third, the exact identification of
the products implicated in side effects claimed for medicinal plants is usually lack-
ing. In spite of these inadequacies, there are a number of general comments that
can be made with regard to avoiding potential serious side effects from herbal
medicines.
The definition of “toxic” is ultimately a matter of viewpoint. Traditionally, herbs
and herbal products have been considered to be nontoxic and have been used by
the general public and traditional medicinal doctors worldwide to treat a range of
ailments. The fact that something is natural does not necessarily make it safe or ef-
fective. The active ingredients of plant extracts are chemicals that are similar to
those in purified medications, and they have the same potential to cause serious
adverse effects. Whilst the literature documents severe toxicity resulting from the
use of herbs, on many occasions the potential toxicity of herbs and herbal products
has not been recognized [108]. In certain countries, such as Taiwan, herbs can be
obtained from temples, night markets, street vendors, herbal stores, neighbor-
hoods, or relatives, and from traditional medicine practitioners. Ordinary people
recommend the medicines to others without safety considerations. The general
public and many practitioners also believe that the herbs are nontoxic. Apparently,
this cultural style/concept needs more attention in terms of drug safety education.
Herbs and herbal preparations can cause toxic adverse effects, serious allergic re-
actions, adverse drug interactions, and can interfere with laboratory tests
High-risk patients such as the elderly, expectant mothers, children,
those taking several medications for chronic conditions, those with hypertension,
depression, high cholesterol or congestive heart failure, should be more cautious
in taking herbal medicine.
It is axiomatic that pregnancy should be a time of minimal medical intervention,
and herbalists in particular regard pregnancy as a “contraindication” to taking her-
bal medicines
Two kinds of side effects have been reported for herbal medicines. The first, con-
sidered to be intrinsic to herbal drugs themselves, is mainly related to predictable
toxicity due to toxic constituents of the herbal ingredients and overdosage, and the
second is allergy. Many cases of allergic reactions have been reported for herbal
drugs. It is impossible to completely eliminate the possibility of any substance, in-
cluding prescription drugs, herbal remedies, or cosmetics, producing an allergic
response in people exposed to them. Herbal medicines do not present any more of
a problem in this respect than any other class of widely used foods or drugs.
Based on published reports, the side effects or toxic reactions associated with
herbal medicines in any form are rare. This could be due to the fact that herbal
2.7 Toxicity of Herbal Drugs 43
medicines are generally safe, that adverse reactions following their use are under-
reported, or because the nature of the side effects or minor allergic reactions are
such that they are not reported.
Perhaps the major problem with regard to the safety of herbal medicines is relat-
ed to the manufacturing practice, including contamination, substitution, incorrect
preparation and dosage, intentional addition of unnatural toxic substances, inter-
actions involving synthetic prescriptions, drugs, and herbal medicines, either in-
tentional or unintentional mislabeling, and the presence of natural toxic contami-
nants. Many ordinary foods contain constituents that could be regarded as poison-
ous. Alpha gliadin produced by gluten in wheat, oats, and rye, the cyanogenic gly-
cosides in many fruit skins and seeds, thiocyanates of the brassica vegetables, and
lectins of many pulses including soya and red kidney bean are such examples.
Cyanogenetic glycodides present in the kernel of many fruits can undergo gastric
hydrolysis, resulting in the release of hydrogen cyanide. Viscotoxins, which are
constituents of mistletoe, are both cytoxic and cardiotoxic [101, 120]. Nonetheless,
these foods are generally regarded as safe. Similarly, both water and oxygen can kill
in excessive amounts! So quantity is often an important consideration.
A number of cases have been reported in the literature in which herbal medi-
cines, used for a number of years with safety, suddenly appear to be unsafe, and to
date there has been no satisfactory explanation for these adverse effects.
In this context herbs can be broadly classified into three major categories:
The food herbs – medicines such as peppermint, ginger, garlic, hawthorn, nettles,
lemon, and balm are gentle in action, have low toxicity, and are unlikely to cause
any adverse response. They can be consumed in substantial quantities over long
periods of time without any acute or chronic toxicity. However they may bring
about allergic reactions in certain individuals.
The medicinal herbs – these are not daily “tonics” and need to be used with great-
er knowledge (dosage and rationale for use) for specific conditions (with a med-
ical diagnosis) and usually only for a limited period. They have a greater poten-
tial for adverse reactions and in some cases drug interactions. They include aloe
vera, black cohosh, comfrey, echinacea, ephedra, ginkgo biloba, ginseng, kava
kava, milk thistle, and senna.
The poisonous herbs have a strong potential for either acute or chronic toxicity and
should only be prescribed by trained clinicians who understand their toxicology
and appropriate use. Fortunately, the vast majority of these herbs are not avail-
able to the public and are not sold in health food or herbal stores. Aconite, Arni-
ca spp., Atropa belladonna, digitalis, datura, male fern, gelsemium, and veratrum
are some examples [116].
There are herbs such as Lobelia and Euonymus spp. that have powerful actions,
often causing nausea or vomiting, although they are safe under appropriate condi-
tions. There is also an idiosyncratic grouping of herbs that have been alleged, with
some scientific support, to exhibit specific kinds of toxicity. The best known exam-
ple is the hepatotoxicity of pyrrolizidine alkaloid-containing plants such as Symphy-
44 2 Quality Control, Screening, Toxicity, and Regulation of Herbal Drugs
tum (comfrey), Dryopteris (male fern), Viscum (mistletoe), and Corynanthe (Yohim-
be)
For several reasons it is not possible to establish absolute safety standards for her-
bal preparations based solely on epidemiological studies. First, these types of stud-
ies would be costly. Second, there is little published data in countries where the
major use of medicinal plants occurs and thus general standards based on a limit-
ed number of reports would have little meaning. Third, the exact identification of
the products implicated in side effects claimed for medicinal plants is usually lack-
ing. In spite of these inadequacies, there are a number of general comments that
can be made with regard to avoiding potential serious side effects from herbal
medicines.
The definition of “toxic” is ultimately a matter of viewpoint. Traditionally, herbs
and herbal products have been considered to be nontoxic and have been used by
the general public and traditional medicinal doctors worldwide to treat a range of
ailments. The fact that something is natural does not necessarily make it safe or ef-
fective. The active ingredients of plant extracts are chemicals that are similar to
those in purified medications, and they have the same potential to cause serious
adverse effects. Whilst the literature documents severe toxicity resulting from the
use of herbs, on many occasions the potential toxicity of herbs and herbal products
has not been recognized [108]. In certain countries, such as Taiwan, herbs can be
obtained from temples, night markets, street vendors, herbal stores, neighbor-
hoods, or relatives, and from traditional medicine practitioners. Ordinary people
recommend the medicines to others without safety considerations. The general
public and many practitioners also believe that the herbs are nontoxic. Apparently,
this cultural style/concept needs more attention in terms of drug safety education.
Herbs and herbal preparations can cause toxic adverse effects, serious allergic re-
actions, adverse drug interactions, and can interfere with laboratory tests
High-risk patients such as the elderly, expectant mothers, children,
those taking several medications for chronic conditions, those with hypertension,
depression, high cholesterol or congestive heart failure, should be more cautious
in taking herbal medicine.
It is axiomatic that pregnancy should be a time of minimal medical intervention,
and herbalists in particular regard pregnancy as a “contraindication” to taking her-
bal medicines
Two kinds of side effects have been reported for herbal medicines. The first, con-
sidered to be intrinsic to herbal drugs themselves, is mainly related to predictable
toxicity due to toxic constituents of the herbal ingredients and overdosage, and the
second is allergy. Many cases of allergic reactions have been reported for herbal
drugs. It is impossible to completely eliminate the possibility of any substance, in-
cluding prescription drugs, herbal remedies, or cosmetics, producing an allergic
response in people exposed to them. Herbal medicines do not present any more of
a problem in this respect than any other class of widely used foods or drugs.
Based on published reports, the side effects or toxic reactions associated with
herbal medicines in any form are rare. This could be due to the fact that herbal
2.7 Toxicity of Herbal Drugs 43
medicines are generally safe, that adverse reactions following their use are under-
reported, or because the nature of the side effects or minor allergic reactions are
such that they are not reported.
Perhaps the major problem with regard to the safety of herbal medicines is relat-
ed to the manufacturing practice, including contamination, substitution, incorrect
preparation and dosage, intentional addition of unnatural toxic substances, inter-
actions involving synthetic prescriptions, drugs, and herbal medicines, either in-
tentional or unintentional mislabeling, and the presence of natural toxic contami-
nants. Many ordinary foods contain constituents that could be regarded as poison-
ous. Alpha gliadin produced by gluten in wheat, oats, and rye, the cyanogenic gly-
cosides in many fruit skins and seeds, thiocyanates of the brassica vegetables, and
lectins of many pulses including soya and red kidney bean are such examples.
Cyanogenetic glycodides present in the kernel of many fruits can undergo gastric
hydrolysis, resulting in the release of hydrogen cyanide. Viscotoxins, which are
constituents of mistletoe, are both cytoxic and cardiotoxic [101, 120]. Nonetheless,
these foods are generally regarded as safe. Similarly, both water and oxygen can kill
in excessive amounts! So quantity is often an important consideration.
A number of cases have been reported in the literature in which herbal medi-
cines, used for a number of years with safety, suddenly appear to be unsafe, and to
date there has been no satisfactory explanation for these adverse effects.
In this context herbs can be broadly classified into three major categories:
The food herbs – medicines such as peppermint, ginger, garlic, hawthorn, nettles,
lemon, and balm are gentle in action, have low toxicity, and are unlikely to cause
any adverse response. They can be consumed in substantial quantities over long
periods of time without any acute or chronic toxicity. However they may bring
about allergic reactions in certain individuals.
The medicinal herbs – these are not daily “tonics” and need to be used with great-
er knowledge (dosage and rationale for use) for specific conditions (with a med-
ical diagnosis) and usually only for a limited period. They have a greater poten-
tial for adverse reactions and in some cases drug interactions. They include aloe
vera, black cohosh, comfrey, echinacea, ephedra, ginkgo biloba, ginseng, kava
kava, milk thistle, and senna.
The poisonous herbs have a strong potential for either acute or chronic toxicity and
should only be prescribed by trained clinicians who understand their toxicology
and appropriate use. Fortunately, the vast majority of these herbs are not avail-
able to the public and are not sold in health food or herbal stores. Aconite, Arni-
ca spp., Atropa belladonna, digitalis, datura, male fern, gelsemium, and veratrum
are some examples [116].
There are herbs such as Lobelia and Euonymus spp. that have powerful actions,
often causing nausea or vomiting, although they are safe under appropriate condi-
tions. There is also an idiosyncratic grouping of herbs that have been alleged, with
some scientific support, to exhibit specific kinds of toxicity. The best known exam-
ple is the hepatotoxicity of pyrrolizidine alkaloid-containing plants such as Symphy-
44 2 Quality Control, Screening, Toxicity, and Regulation of Herbal Drugs
tum (comfrey), Dryopteris (male fern), Viscum (mistletoe), and Corynanthe (Yohim-
be)
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