The document defines antidotes as therapeutic substances that counteract the toxic actions of xenobiotics. Antidotes are classified based on their mode of action into physiological, physical, and chemical antidotes. Physiological antidotes counteract poisons through opposing pharmacological effects, while physical antidotes interfere with poisons through adsorption, coating, or dissolving. Chemical antidotes specifically interact with or neutralize toxins through complex formation or metabolic conversion into less toxic products. Common ways antidotes work include inert complex formation, accelerated detoxification, reduced toxic conversion, receptor site competition, receptor site blockage, and bypassing toxic effects.

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ANTIDOTES
T.SOUJANYA
PHARM.D
16DC1T0021

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ANTIDOTES
Definition: According to WHO, an antidote is defined as a therapeutic substance
used to counteract the toxic actions of a specified xenobiotic. Antidotes reduce the
overall burden of health service in managing of poisoning cases.
The term antidote is a Greek word “Antididonai” meaning “given against”. In
Cambridge dictionary, antidote is defined as a chemical especially a drug that limits
the effects of a poison or a way of preventing or acting against something bad.
Poison + antidote = destroy poisoning
Classification of antidote: According to mode of action, they are classified as
follows:
1. Physiological/pharmacological antidote
2. Physical antidote
3. Chemical antidote
1. Physiological/pharmacological antidote: It counteracts the effects of a poison
by producing the opposite pharmacological effects.
e.g.: ACHE inhibitor morphine
Pharmacological antidotes may neutralize or antagonize the effects of a toxicant.
This type of antidote may act by following 5 mechanisms:
• Preventing the formation of toxic metabolites.
• By facilitation of more rapid or complete elimination of a toxicant.
• By competing with the toxicant’s action at a receptor site.
• By blocking receptors responsible for the toxic effect.
• By aiding in the restoration of normal function.
2. Physical antidote: The agent which is used to interfere with poison through
physical properties, not change their nature is known as physical antidote.

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a) Adsorbing: Adsorption is the adhesion of atoms, ions or molecules from a
gas, liquid or dissolved solid to a surface. This process creates a film of the
adsorbate on the surface of the adsorbent.
e.g.: Activated charcoal (Universal antidote)
b) Coating: A mixture of egg and milk make a coat over the mucosa.
c) Dissolving: 10% alcohol or glycerine for carbolic acid.
3. Chemical antidote: It interacts specifically with a toxicant or neutralize the
toxicant.
e.g.: Metal chelators combine with metals to form complexes that can then be
eliminated by the kidneys.
It mainly act by 2 mechanisms:
i) Complex formation.
ii) Metabolic conversion.
i) Complex formation: antidote make complex with the toxicant making it
unavailable to cross the membrane or to interact with the receptors.
e.g.: DMSA (dimercaprol and dimercaptosuccinic acid are sulfhydryl compounds
that bind metals such as arsenic acid, lead).
ii) Metabolic conversion:
Detoxification to less toxic product: Nitrate interact with haemoglobin and cyanide
to form cyanomethaemoglobin, which is less toxic than cyanide and interfere with
the cyanide access to cytochrome oxidase system.
Nitrite + oxyhaemoglobin methaemoglobin + Cyanide
cyanomethaemoglobin
Antidote administration: Antidotes work in any one of a no. of ways. Common
modes of action are as follows:
1. Inert complex formation.
2. Accelerated detoxification.
3. Reduced toxic conversion.

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4. Receptor site competition.
5. Receptor site blockage.
6. Toxic effect bypass.
1. Inert complex formation: Some antidotes interact with the poison to form an
inert complex which is then excreted from the body.
e.g.: a) Chelating agents for heavy metals.
b) Prussian blue for thallium.
c) Dicobalt edetate for cyanide etc.
2. Accelerated detoxification: Some antidotes accelerate the detoxification of a
poison.
e.g.: Acetyl cysteine acts as a glutathione substitute which combines with
hepatotoxic paracetamol metabolites and detoxifies them.
Thiosulfate accelerates the conversion of cyanide to nontoxic thiocyanate.
Cyanide thiosulfate thiocyanate (detoxification)
(toxic) (antidote) (nontoxic)
3. Reduced toxic conversion: Antidote reduce the conversion of toxic product.
e.g.: Ethanol which inhibits the metabolism of methanol to toxic metabolites by
competing for the same enzyme (alcohol dehydrogenase).
methanol
alcohol dehydrogenase
formaldehyde
alcohol dehydrogenase
formic acid
folate
Co2 and H2O

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4. Receptor site competition: Some antidotes displace the poison from specific
receptor sites, thereby antagonizing the effects completely.
e.g.: Naloxone which antagonizes the effects of opioids at stereo-specific opioid
receptor sites.
5. Receptor site blockage: Antidote binds to the receptor site of poison and block
its action.
e.g.: This mode of action is best exemplified by atropine which blocks the effects of
anticholinesterase agents such as organophosphates at muscarinic receptor sites.
6. Toxic effect bypass: Antidote bypasses the toxic effect of poison.
e.g.: 100% oxygen in cyanide poisoning.

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