Why was cyanide a common poison?

Why was cyanide Agatha Christie's poison of choice?
04 February 2020


A red apple in a tree



Why was cyanide a common poison?


We put Adam's question to forensic toxicologist Lorna Nisbet...

Lorna - Well, I can't say specifically what was going through Agatha Christie's mind when she decided to use cyanide so much, but she was a trained pharmacist. So she did have a pharmaceutical background, which is why she used so many poisons in her books. But I do have a question for the panel, which is how many of Agatha Christie's character's did die from cyanide poisoning?

Matt - I feel like zero would be a very QI type answer.

Lorna - No, it's not. It's not zero.

Sam - Sadly, I need my wife here, she's a massive Agatha Christie fan and I am not. So I'm going to guess. I'm going to go with three.

Lorna - Okay. She actually killed 18 characters in total using cyanide. And so cyanide is in 10 of her books and 4 of her short novels. But cyanide has all these kind of historical connotations and it's not actually as common as what you would believe. It's just unfortunately that it's been used in some really horrible events throughout history.

Chris - How does it work, Lorna? What does cyanide do to you to kill you?

Lorna - It just slows down your respiratory system, so it's to do with your blood and it binds with your blood and you can't breathe any more effectively. But it's all to do with as well how the cyanide is packaged as to how good it's going to be within your bloodstream. When you're talking about cyanide, you're just talking about a carbon and nitrogen functional group. So if you're looking at sodium cyanide or potassium cyanide for example, it's about the strength of the bond between whatever else it's attached to and that CN. That's why hydrogen cyanide is so powerful, because the bond between the hydrogen and the cyanide group is so weak.

Chris - So it's happy to surrender its cyanide onto parts of your body and disable them?

Lorna - Yes, exactly. That are antidotes and cures for cyanide poisoning now, but obviously you have to be extremely fast-acting to do that.

Chris - So it is a good choice of way to get rid of someone then?

Lorna - It is relatively quick acting, but you wouldn't really want to use that. In nowaday's terms you could always detect it regardless. In a lot of Agatha Christie's novels, she would see that it would taste of bitter almonds and it's actually that bitter almond smell of cyanide rather than the other way round.

Chris - They do contain it, don't they?

Lorna - Yes they do. Lots of different things in nature contain cyanide. So apple pips, for example, will contain it and apricot pips, but you do need to have a ridiculous amount before it's going to do you any harm.

Chris - I think we calculated you'd have to eat something like your own body weight in apple pips, in apples rather, to get enough amygdalin, which is the cyanide source in apple pips, in order to actually compromise yourself. So you'd probably die of poisoning from over-eating apples before you actually died of cyanide poisoning, I think was our conclusion. Do you think you're on safe territory then? Eat your apple core and you're probably ok?

Lorna - I don't think you have to sit there and go, "Oh, I swallowed an apple pip. I'm now going to die." You don't need to go to A&E for that one. You'll be OK


Dear Lorna

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