Classification of Poisons by Origin, Action and Chemical Nature

The oldest working classification splits poisons by their natural source. A toxicologist reaches for it first because origin is what the field history usually surrenders. The four buckets are mineral, vegetable, animal and synthetic, and each one has a recognisable Indian casebook.

Mineral poisons are the toxic metals and a small group of non-metals. Arsenic in groundwater across West Bengal, Bihar and Assam, lead in surma and old paint, mercury in vermilion and amalgam, and yellow phosphorus from rodenticide paste and firecracker manufacture are the four that an Indian state FSL sees most often. The Reinsch test, in which a copper foil is dipped into acidified gastric contents and turns black if arsenic, mercury, antimony or bismuth is present, is the classical screening tool that still appears in CFSL Hyderabad and SFSL bench notes. The Marsh test, which converts arsenic to arsine gas and deposits a metallic mirror on a heated glass tube, is the historical confirmatory and the one that finished the Lafarge case in 1840.

Vegetable poisons cover the alkaloids and the cardiac glycosides. Strychnine from Strychnos nux-vomica, opium and its morphine and codeine alkaloids, atropine and hyoscyamine from Datura stramonium and D. metel, ricin from castor seed, abrin from Abrus precatorius, and the glycosides digitalin from Digitalis and oleandrin from Cerbera odollam are all part of Indian medico-legal practice. Cerbera odollam in Kerala, often called the suicide tree, accounted for roughly 49 deaths annually across Kerala between 1989 and 1999, with the highest burden recorded in the southern districts of Trivandrum and Kollam.

Animal poisons include the venoms of the Big Four Indian snakes (cobra, krait, Russell's viper, saw-scaled viper), scorpion venom from Hottentotta tamulus, hymenoptera stings, and marine toxins like tetrodotoxin (occasionally seen in pufferfish from Andhra and Tamil Nadu coast) and ciguatera. Synthetic poisons are the modern dominators: barbiturates and benzodiazepines on the pharmaceutical side, organophosphates (chlorpyrifos, monocrotophos, dimethoate) on the agricultural side, and aluminium phosphide on the grain-storage side.

The action-based classification is the one a clinician and a post-mortem surgeon use in real time. It groups poisons by what they do to tissue, not by what they are made of. Five working categories cover the bulk of Indian casework: corrosives, irritants, neurotoxic, cardiotoxic and asphyxiants. A miscellaneous bucket holds the rest.

Corrosives destroy tissue at the point of contact. Concentrated sulphuric acid, hydrochloric acid and nitric acid on the acid side, sodium hydroxide and potassium hydroxide on the alkali side, and phenol as the classical organic corrosive. The post-mortem picture is unmistakable: blackened, charred lips and mouth for sulphuric, yellow staining for nitric, soapy mucosa for alkalis. The 2013 Mumbai acid-attack jurisprudence and the BNS sections that replaced IPC 326A still rely on toxicological identification of the agent.

Irritants inflame the gut without dissolving it. Arsenic trioxide, antimony, copper sulphate (the classical "tooti" used historically in homicidal cases in north India), zinc phosphide and yellow phosphorus all sit here, alongside mechanical irritants like ground glass and chemical irritants from plant latex such as Calotropis. The cardinal clinical sign is the cholera-like syndrome of profuse vomiting, rice-water stools, abdominal cramps and dehydration.

Neurotoxic poisons subdivide by site of action. Cerebral neurotoxins like ethanol, methanol and opioids work on the brain. Spinal neurotoxins like strychnine and gelsemine cause the classical opisthotonus with retained consciousness, a presentation that is clinically unmistakable once seen. Peripheral neurotoxins include curare-type non-depolarising blockers and the anticholinergic atropine and hyoscyamine from Datura, behind a long history of highway robberies on the Delhi-Agra and Mumbai-Pune corridors where travellers were drugged with datura-laced food.

Cardiotoxins target the heart directly. Digitalis purpurea glycosides, oleandrin from Nerium oleander and Cerbera odollam, and aconitine from Aconitum napellus and the Himalayan Aconitum species are the three that India sees. The classical sign is the inverted T-wave with bradyarrhythmia leading to ventricular fibrillation.

Asphyxiants split into simple and chemical. Simple asphyxiants like CO2 in confined spaces (manhole and septic-tank deaths in Delhi, Bengaluru and Chennai sanitation work) displace oxygen without binding anything. Chemical asphyxiants bind: carbon monoxide to haemoglobin, hydrogen cyanide and hydrogen sulphide to cytochrome oxidase. Aluminium phosphide releases phosphine, which is a mitochondrial poison that behaves like a chemical asphyxiant at the cellular level.

1. 1. Toxidrome at the bedside
   ICU registrar matches signs (pupils, secretions, bowel sounds, mental status, vitals) to a toxidrome: cholinergic, anticholinergic, sympathomimetic, sedative-hypnotic, opioid or asphyxiant pattern.
2. 2. Action class assigned
   The toxidrome maps onto an action class. Cholinergic plus diaphoresis on a farmer points to organophosphate (neurotoxic peripheral). Garlic breath plus shock points to aluminium phosphide (chemical asphyxiant).
3. 3. Antidote started empirically
   Atropine and pralidoxime for OP, magnesium for AlP, naloxone for opioids. No waiting for FSL confirmation, the toxidrome is enough.
4. 4. Sample preservation matched to class
   Volatile asphyxiants need sealed blood in saturated NaF; alkaloid neurotoxins go into rectified spirit; corrosive cases need scene-sample acid identification before viscera.

Once the case reaches the toxicology lab, the action label is no longer enough. The bench needs a chemical category because the chemistry decides the isolation method. Four buckets cover almost everything: inorganic, organic, gaseous and biological.

Inorganic poisons include the strong acids and alkalis, the metallic elements and their salts (arsenic trioxide, lead acetate, mercuric chloride, copper sulphate, thallium sulphate, antimony tartrate), and inorganic anions like cyanide, sulphide, fluoride and nitrite. The extraction stack is wet digestion with nitric and sulphuric acid or microwave-assisted digestion, followed by AAS, ICP-OES or ICP-MS for metals and ion chromatography for anions.

Organic poisons cover the alkaloids (strychnine, morphine, atropine, nicotine, aconitine), the glycosides (digoxin, oleandrin), the hydrocarbons (kerosene, petrol, benzene, toluene), and the synthetic drugs (barbiturates, benzodiazepines, tricyclic antidepressants, antipsychotics, opioids, organophosphates, organochlorines). The classical isolation is Stas-Otto acid-alkaline partitioning, the modern equivalent is solid-phase extraction (SPE) followed by HPLC, LC-MS/MS or GC-MS.

Gaseous poisons are carbon monoxide, hydrogen cyanide, hydrogen sulphide, phosphine (from aluminium phosphide), chlorine, ammonia and the volatile organic compounds. Conway microdiffusion, headspace GC and direct spectrophotometry of carboxyhaemoglobin are the standard isolation and detection routes.

Biological poisons are toxins of plant, animal or microbial origin where the active species is a protein or peptide rather than a small molecule. Snake venoms, ricin, abrin, botulinum toxin and the staphylococcal enterotoxins sit here. ELISA and LC-MS/MS proteomics have largely replaced the older biological assays at reference centres.

The fourth classification is not chemical at all. It is the manner of poisoning, and it carries the case into the criminal courts. Three buckets cover almost every Indian medico-legal poisoning: accidental, suicidal and homicidal. The IO and the magistrate decide the label, the toxicologist supplies the evidence that supports or excludes each option.

Accidental poisoning is the largest bucket by absolute numbers in NCRB Accidental Deaths and Suicides in India (ADSI). The Indian patterns are well known: kerosene ingestion by toddlers below age five in homes where it is decanted into soft-drink bottles, organophosphate spray drift onto agricultural workers in Andhra Pradesh and Telangana, methanol in hooch tragedies (Bihar 2022, Gujarat 2009, Tamil Nadu 2023), copper sulphate and zinc phosphide ingestion mistaken for sugar or salt, and snake bite estimated by the ICMR Million Death Study (Mohapatra et al. 2011) at roughly 46,000 deaths per year, with a later nationally representative study (Sharma et al. 2020, eLife) revising the 2000-2019 average upward to approximately 58,000 per year.

Suicidal poisoning is the bucket that dominates NCRB ADSI for the working age group. Aluminium phosphide tablets ("rice tablets", Celphos, Phostoxin) in Punjab, Haryana and western UP, organophosphate concentrates in rural Maharashtra, Andhra and Karnataka, paracetamol and sedative overdoses in urban India, yellow oleander seeds and Cleistanthus collinus (Oduvanthalai) decoction in coastal Kerala, Tamil Nadu and Andhra, and copper sulphate in pockets of Karnataka. Suicidal poisons share a profile: easy to obtain, lethal in a small visible dose, and culturally legible to the deceased.

Homicidal poisoning is rare but never zero. The ideal homicidal poison is tasteless, odourless, colourless, cheap, mimics a natural disease and is hard to detect. Arsenic mixed into food, thallium in tea (the IISc 2016 case and the older Tipu-era allegations), datura in highway food and drink, and sleeping pill cocktails in suspected serial cases are the recurring Indian patterns. The toxicologist's contribution is the time-of-death window, the dose-response and the matrix distribution that distinguishes a single fatal dose from chronic administration.

A working forensic toxicologist treats all four classifications as a single decision tree. The autopsy surgeon's note that the deceased was a 35-year-old farmer from Beed district who collapsed in his field after fenitrothion spraying immediately fixes the origin (synthetic agricultural), the action (peripheral neurotoxic, cholinergic), the chemical nature (organophosphate ester) and the most probable intent (accidental, occupational). The toxicology requisition that follows asks for plasma cholinesterase, RBC acetylcholinesterase and GC-MS confirmation of the parent compound and its dialkyl phosphate metabolites in urine.

Compare that to a 19-year-old college student from Patiala who is brought to PGI Chandigarh with garlic-smelling breath, refractory shock and a tablet wrapper in the pocket. The same four-way classification reads: synthetic (industrial fumigant), chemical asphyxiant (phosphine on mitochondrial cytochrome oxidase), inorganic phosphide (AlP), suicidal (the most common Punjab pattern in the 15 to 30 age bracket per PGI's own published series). The toxicology workflow is silver nitrate paper for phosphine in gastric headspace, AAS or ICP for aluminium, and a search for the foil wrapper at scene by the IO.

Classification also rules out branches the lab does not need to chase. A blackened mouth with sulphuric acid odour and acid-burn pattern on the oesophagus stops the toxicologist from running a Stas-Otto extraction for alkaloids on viscera, because the chemistry tells her the cause of death is already obvious and the scene needs an acid identification (specific gravity, pH, anion test) rather than a viscera workup. The most expensive mistake in toxicology is the unfocused screen, and classification is the cheapest filter against it.