Question: What are the most poisonous plants for a human being? Why are they so dangerous? How do we know which quantity of the toxin can be lethal and how much to use as a medicine (like a vaccine)?

Jennifer Bates answered on 16 Nov 2016:

Hi,
Ooh, this is fun, I get to write a list of freaky plants!

First though:
How do we define most poisonous? Are we going by number of deaths per year? Or just how long it takes to die if you eat/touch it? Let’s go with eat/touch because the other one depends on lots of factors like how easy it is to get hold of it, people reporting deaths, where in the world we are talking about etc…

I’d probably say things in the Solanaceae family are high up on that list. They contain a large range of nasty alkaloids, from nicotine, which in pure form is lethal (but not in the distilled form found in nictoine patches etc…fret not, but avoid cigarettes, nicotine patches etc, for your health in general), to solanine, a toxic glycoalkaloid found in things like some nightshades or tropane, found in belladonna (deadly nightshade). That said, this family also contains potatoes and chillies, and one of the tropanes, Scopolamine, is used as an anti-sickness for chemotherapy patients, so don’t write the whole family off! For the chemicals solanine you need about 2 and 5 mg per kg of body weight which is actually quite a lot when you think about the size of a nightshade seed! And as someone that as a very small child accidentally ate this, I can say with some knowledge – you can survive, if you get medical help! (NOTE – if you see nightshade in the garden, get gloves before you pull it up, the chemicals are in the stems, and make you vomit. And never leave it in the garden in case children find it, the berries look pretty and kids can accidentally eat them – the evidence of this is typing this response!!!).

Probably something like Ricinus communis should go on the list – this is contains ricin, which is lethal. 4-8 seeds will kill. Again though, this plant that is used to make castor oil, so positive things from nasty plants!

Foxgloves are fairly evil too, containing digoxin, which is dangerous because it can stop the heart in high/unregulated doses, but is also useful because it is now used in heart medicine to regulate heartbeats!

And any scary list has to have the Hippomane mancinella, or ‘little apple of death’ on it! This plant has the powerful irritant phorbol and several other chemicals in it, which when you even touch it cause a violent reaction on the skin. Even standing under it during rain causes blisters!

Okay, you’ve clearly got me onto a pet topic and even I am beginning to scare myself (although I hope you note that most of these have positives too – go plants!), so let’s move on, shall we, to the other bits of your question…

Quantity of toxins that are lethal – I’d say probably a long history of trial and error by the human species sadly. We’ve around a long time and there is a reason for that – we have very good memories! We tend to note things like, this tree is trying to kill you, stay away, or if you eat ten of those you are going to puke, maybe don’t eat them…

But the same goes for medicinal properties. We’ve been tinkering with plants for health properties for a long time too. There is evidence emerging that people may have been using fern roots for several hundred thousand years as vermifuges (worm-removers), and often you find strange seeds turning up on archaeological sites that have less nutritional value but possible medicinal value.

Today we can extract the chemicals and run in vitro (test-tube) experiments to see what the bioactive substances in plants do, like the tests to see if nicotine is actually carcinogenic for example. There are a lot more regulations today than just, hey watch Bob over there as he eats that berry, it might be useful for us…

Thanks for this question, it was fun to write the answer, even though I must admit, plants are now looking a bit more scary…!