Forensic Medicine and Toxicology
Chemical Toxicology: Corrosives, Metals and Gases
Chemical toxicology for MBBS and NEET-PG: corrosives, lead, arsenic and mercury, methanol, carbon monoxide and cyanide with their antidotes, mapped to NMC codes FM9.1 to FM9.6.
MedNext Academy | 3 min read
Chemical Toxicology: Corrosives, Metals and Gases
Chemical toxicology for MBBS and NEET-PG: corrosives, lead, arsenic and mercury, methanol, carbon monoxide and cyanide with their antidotes, mapped to NMC codes FM9.1 to FM9.6.
This chapter covers the inorganic and chemical poisons: the corrosives, the toxic metals such as lead, arsenic and mercury, the alcohols including methanol, and the toxic gases such as carbon monoxide and cyanide. For each it links the mechanism to the clinical picture, the diagnostic clues and the specific antidote.
High-yield: Chemical Toxicology: Corrosives, Metals and Gases
- Corrosive acids cause coagulative necrosis with dry, hard eschars, while alkalis cause liquefactive necrosis that penetrates more deeply.
- Oxalic acid poisoning causes hypocalcaemia, and calcium gluconate is the antidote.
- Lead poisoning causes a punctate basophilic stippling of red cells, a blue lead line on the gums (Burton's line) and wrist drop from motor neuropathy.
- Chelating agents used in lead poisoning include calcium disodium EDTA, DMSA (succimer) and D-penicillamine.
- Acute arsenic poisoning causes rice-water stools and shock, and chronic exposure causes hyperkeratosis, raindrop pigmentation and Mees' lines on the nails.
- Arsenic and mercury poisoning are treated with the chelator dimercaprol (British anti-Lewisite).
- Mercury vapour and organic mercury damage the nervous system, causing tremor, erethism and the sensory changes of Minamata disease.
- Iron overdose is treated with the chelator desferrioxamine.
- Methanol is metabolised to formic acid, causing high anion gap metabolic acidosis and blindness from optic nerve damage.
- Ethanol and fomepizole are antidotes for methanol and ethylene glycol as they block alcohol dehydrogenase.
- Ethylene glycol poisoning produces calcium oxalate crystals in the urine and acute renal failure.
- Carbon monoxide binds haemoglobin about 240 times more avidly than oxygen and is treated with high-flow or hyperbaric oxygen.
- Cyanide blocks cytochrome oxidase, causing a bright red venous blood and rapid death; antidotes include hydroxocobalamin and sodium thiosulphate.
- Aluminium and zinc phosphide release phosphine gas and have no specific antidote, so management is supportive.
- Paraquat ingestion causes progressive pulmonary fibrosis, and supplemental oxygen worsens the lung injury, so it is avoided.
Metals, alcohols, gases and their antidotes
- **Lead:** Basophilic stippling, Burton's line, wrist drop. Chelate with calcium disodium EDTA, DMSA or D-penicillamine.
- **Arsenic and mercury:** Rice-water stools and pigmentation (arsenic); tremor and neuropathy (mercury). Antidote dimercaprol.
- **Methanol and ethylene glycol:** High anion gap acidosis; blindness or oxalate renal failure. Block with ethanol or fomepizole.
- **Cyanide:** Blocks cytochrome oxidase, bright red venous blood. Antidotes hydroxocobalamin and sodium thiosulphate.
NMC competencies in this chapter
- **FM9.1:** Corrosives, phenol, oxalic acid and salicylates
- **FM9.2:** Phosphorus, iodine and barium
- **FM9.3:** Metal poisoning: lead, arsenic, mercury, iron, copper, cadmium and thallium
- **FM9.4:** Ethanol, methanol and ethylene glycol
- **FM9.5:** Pesticides, paraquat and phosphide fumigants
- **FM9.6:** Toxic gases: ammonia, carbon monoxide, cyanide and tear gases
Frequently Asked Questions
Why does methanol cause blindness?
Methanol is broken down to formic acid, which damages the optic nerve and retina and also causes a severe high anion gap metabolic acidosis. Ethanol or fomepizole blocks this conversion.
What is the antidote for arsenic and mercury poisoning?
Dimercaprol, also known as British anti-Lewisite, is the classic chelating antidote for both arsenic and inorganic mercury poisoning.
How does cyanide kill so rapidly?
It blocks cytochrome oxidase, stopping cellular use of oxygen. The blood stays oxygen-rich and bright red while the tissues suffocate, leading to rapid collapse and death.
Why is oxygen used with caution in paraquat poisoning?
Paraquat drives oxidative lung injury, and extra oxygen generates more free radicals that accelerate the pulmonary fibrosis, so supplemental oxygen is given only for genuine hypoxia.
Continue reading
MBBSAll Forensic Medicine and Toxicology chapters
Continue through the Forensic Medicine and Toxicology chapter map.
Continue studying Forensic Medicine and Toxicology
Explore clinician-written learning resources, structured revision and practice across the MedNext platform.
Open in the MedNext appSee plans

