This article throws light upon the four major causes of the problems with toxicity of herbal medicines. The causes are: 1. Herbal Material Adulterated with Toxic Non-Plant Materials 2. Herbal Material Adulterated with other Plant 3. Classification of Plant Species of Proven Toxicity 4. Interactions between the Herbal Medicines and Orthodox Drugs or other Substances.
Cause # 1. Herbal Material Adulterated with Toxic Non-Plant Materials, e.g. Lead:
It is well known that there are many contaminants and residues that may cause toxicity problems in consumers of herbal medicines. Many are natural toxic metals or bacteria. Some arise from past or present use of agents or materials that pollute the environment and subsequently medicinal plants, such as emissions from factories or the residues of certain pesticides.
The contamination of herbal medicines with heavy metals/toxic elements continues to be a major international problem. A comprehensive review in 1992 summarized test results on products and case histories of patients who had experienced toxic effects as a result of the presence of heavy metals in herbal preparations.
Similar findings continue to be reported and the potential impact on public health is significant. In roots cases involving synthetic drugs, these are undeclared in the product and only come to light when the user experiences adverse effects which are sufficiently serious to warrant medical intervention.
Exposure to the undeclared drug is revealed in the subsequent investigation of clinical cases. Of particular concern is the deliberate addition of closely related derivatives of pharmaceutical drugs, for example the use of nitrosofenfluramine instead of fenfluramine in weight-loss products.
The situation with the heavy metals/toxic elements in herbal products differs in that whilst these ingredients may arise from the plant ingredients themselves or be introduced as trace contaminants during processing, they are also frequently added intentionally and declared as ingredients within some Traditional Chinese Medicine (TCM) and Asian medicine formulations.
The Chinese Pharmacopoeia, for example, includes monographs for realgar (arsenic disulfide), calomel (mercurous chloride), cinnabaris (mercuric sulfide) and hydrargyri oxydum rubrum (red mercuric oxide), and includes formulations for nearly 50 products that include one or more of these substances.
Recent research has also demonstrated that herbs may absorb heavy metals during growth. For these reasons, there is currently a potential global danger to the health and well-being of people where these practices are encouraged.
This risk can be reduced by ensuring that herbal medicines with harmful contaminants and residues do not reach the public, by assessing the quality of the medicinal plants, herbal materials and finished herbal products before they reach the market.
In all situations, the acceptable daily intake (ADI) of a toxic chemical, heavy metal or element estimated as the maximum amount of an agent, expressed on a body mass basis, to which an individual in a population may be exposed daily over his or her lifetime without appreciable health risk has to be critically investigated during quality control assessment of herbal medicines.
Cause # 2. Herbal Material Adulterated with other Plant:
Cases of serious cardiac arrhythmias were reported in the USA in 1997 following the accidental substitution of plantain with Digitalis lanata. Subsequent investigation revealed that large quantities of the contaminated plantain had been shipped to more than 150 manufacturers, distributors and retailers over a two-year period.
Fourteen cases of podophyllum poisoning have been reported from Hong Kong following the inadvertent use of the roots Podophyllum hexandrum instead of Centiana and Clematis species. It is reported that this accidental substitution arose because of the apparent similarity in morphology of the roots.
Cases of cardio toxicity resulting from the ingestion of Aconitum species used in TCM have been reported from Hong Kong. In TCM, Aconitum rootstocks are processed by soaking or boiling them in water in order to hydrolyse the aconite alkaloids into their less toxic aconine derivatives.
Toxicity can, however, result when such processes are uncontrolled and un-validated. In the UK, the internal use of aconite is restricted to prescription only.
The dangers of confusing Chinese and Japanese star anise (lllicium anisatum L.), have been known for many years as the dried fruits cannot be distinguished through visual examination. Japanese star anise is similar to the Chinese variety but has been reported to cause neurologic and gastrointestinal toxicities due to the presence of anisatin.
Cause # 3. Classification of Plant Species of Proven Toxicity:
The classification of plants in terms of a hierarchy of toxicity is usually complicated. To simplify this, three working definitions may be set up, making a distinction among the following:
(I) Herbs that are known to contain highly poisonous substances, and are therefore toxic when ingested even in very small amounts. Their use as medicines is strictly controlled e.g., Atropa belladonna (Deadly nightshade) of family Solanaceae
(II) Herbs that contain high amounts of chemical groups known to be toxic if ingested in large amounts. A maximum recommended dose reflects this e.g., Symphytum officinale (comfrey) of family Boraginaceae.
(III) Herbs that are non-toxic even when ingested in large amounts, but in isolated cases may cause hypersensitivity reactions e.g., Althaea officinalis (marshmallow) of family Malvaceae.
Cause # 4. Interactions between the Herbal Medicines and Orthodox Drugs or other Substances:
Interactions occurring between prescribed or over the counter and herbal medicines exist. A systematic review conducted in 2001 of published herb- drug interactions found evidence of interactions involving widely used herbs with prescribed drugs such as warfarin and garlic, St John’s Wort and oral contraceptives. More research has to be encouraged in this area.