Poison and Antidotes


A poison may be defined as any substance administered in whatever way ( by mouth, injection , inhalation , skin ) produces ill health, diseases or death.
Self medication is a major cause of drug poisoning.

Poisoning can be classified as :-
(A) Intentional poisoning – taking substances without intention of causing harm to self. E.g. :- Suicide

(B) Unintentional poisoning – Taking substances without knowing its toxic effects. E.g. :- Accidental

Signs and symptoms of poisoning:-
*Reduced breathing rate
*Nausea *vomiting and diarrhea
*Alteration in heart rate
*Muscle cramps
*Partial Consciousness.

(C) Heavy Metal poisoning –
This poisoning occurs due to intake of salt of As, Pb , Hg or Fe resulting into the toxic effect

Treatment :- Activated charcoal given initially to absorb heavy metal ↓ followed by emetics to eliminate any poison left in the stomach.

(D) Cyanide poisoning :-
This poisoning may occur by inhalation of fumes of hydrocyanic acid (HCN) or inorganic cyanide salt.
Consumption of 300 mg of KCN may cause death.
Treatment :- Sodium nitrite and Sodium Thiosulphate injections as an antidotes.


Antidotes are the substances which react specifically without the ingested poison or toxic substances or with potent drugs in case of overdose.


According to their mechanism of actions, they are classified as:
Physiological Antidotes –
They are antagonists i.e. produce the effect opposite to that of the poison

Antagonists and Physostigmine are two antidotes for each other.
Sodium nitrite in CN- poisoning.

(A) Chemical Antidotes –
They react by combining with the poison and change its chemical nature by converting poison into inactive compounds.

(B) Mechanical Antidotes –

E.g. – Sodium thiosulphate convert toxic cyanide into non-toxic thiocyanate.
EDTA as chelating agent for heavy metal poisoning.

They act by preventing the absorption of poison into the body or expel out the poison by emesis
E.g. :- Activated charcoal absorbs the poison to absorption into intestinal wall.


Syn :- Sodium hyposulfite, Anti-chlor
Preparation –
Prepared by boiling Sodium Sulphite Na2S2O2 with Sulphur (S)
Na2SO3 + S —∆⟶ Na2S2O3 (Sodium thiosulfate)

By reacting sodium hydroxide with Sulphur
6NaOH + 4S ⟶ Na2S2O3 + 2 Na2S + 3H2O

Physical properties –

Large, prismatic crystalline powder
Effervesces in dry air
Practically soluble in H2O and insoluble in alcohol
M.p. -50°c

Chemical properties –

Decomposition of aq. Solution as
4Na2S2O3 ⟶ 3Na2SO4 + Na2S5 ⟶ Na2S + 4S
Sodium thiosulfate Sodium sulfate Sodium sulphide
Barium chloride reacts without Na2S2O3 to give white ppt. of barium thiosulphate
Na2S2O3 + BaCl2 ⟶ BaS2O3↓ + 2NaCl
Barium thiosulphate (ppt.)

Mechanism of antidotes :-

Slow infusion of sodium thiosulfate

react with CN- in the blood

Convert CN- to SCN- (thiocyanate)

SCN- excreted out from the body by kidney.

Na2S2O3 + CN- ⟶ SCN- + Na2S2O3
(Active cyanide) (inactive thiocyanate)
Uses :-
*Used as antidote in cyanide poisoning as iv
*Effective antidote in Pb, Hg and iodine poisoning.
*Use as antioxidant for solution containing iodides
*Standard titrant in Iodimetric analysis.

Dose = 0.3 – 1g (10ml) administrated by i.m and IV. Route
Assay :-
The assay of Na2S2O3 is based on iodometric titrations
In this titration, solution of Na2S2O3 is titrated with Iodine directly (Iode) using starch solution as an indicator.
Excess I2 reacts without starch to give blue colour (end point).
Iodometric Reactions involved :-
Iodometric reaction involved:-
2S2O3-→S4O62-+2e (oxidation)
I2+2e→2I- (Reduction)

Over all reaction:-


Equivalent factor:-

I2≡ 2Na2S2O3

100ml 1M I2≡2*248.2g Na2S2O3
1ml 0.05M I2≡0.02482g Na2S2O3
ACTIVATED CHARCOAL:- It is a dark grey residue consisting of carbon and any remaining ash obtained by removing water and other volatile constitnent.
Burning of wood in absence of air

Residue obtained consists of pure carbon

(Activation of charcoal )
Absorptive powder of charcoal increased by treating it with various substance such as steam, CO2, ZnCl2 at high temperature 500-1000∙c

Charcoal activation results in increase in total surface area

(Fine, black, odourless powder
→Insoluble in H2O and other organic solvent

  1. Emergency antidotes in many forms of poisoning high preventing absorption of poison in intestinal tract.
  2. Used as protective and adsorbent
  3. Due to its high surface area, it is used as a fitter acid
  4. Disinfectant in wounds
  5. Constituent for gum powder
  6. Used in overdose of aspirin


Syn:-Nitrous acid sodium salt
By adding heating sodium nitrate
(Sodium nitrate)
By heating sodium nitrate with lead


Yellow and white crystalline powder.
Freely soluble in H2O and less soluble in alcohol.
On exposure to air, it readily forms sodium nitrate.

Mechanism of antidotes action:-

Injection of NaNO2 causes oxidation of ferrous ion of haemoglobin (Fe2+) into ferric ion (Fe3+) of meth haemoglobin .

Meth haemoglobin combines with serum cyanide and produce cyano-meth haemoglobin.

Prevention of harmful of CN- .

HB (Fe3+) _NaNO2→MeHB (Fe3+)
Meth haemoglobin
Cyano-meth haemoglobin
(Inactive cyanide ion)
Used as antidote in cyanide poisoning.
Vasodilator action due to nitrite ions relax the smooth muscle of blood vessels.
Anti – rust solution : to prevent the rusting of surgical instruments

Doses of injection :- 10-15 ml of 3% solution I.V.