Relating to, containing, or made from arsenic; caused by or resembling arsenic poisoning.
From Latin 'arsenicum' + English suffix -al (meaning 'relating to'). The term became standard in medical and chemical discourse from the Medieval period onward.
Medieval physicians used 'arsenical' as a scary diagnosis, but some even used arsenic medicinally in tiny doses—the line between poison and cure was razor-thin before modern pharmacology understood dosage and toxicity.
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