Cacti
Firstly, for many people, the most dangerous plant to come to mind is a cactus and its spines. While most cacti have spines, many of these spines can be soft and bristly or arranged on the plant body in a way as to not be dangerous or penetrating of skin. Extremes on the other end of the cactus spine scale include long almost nail-hard and nail-thick spines of tall ’tree-like’ cacti e.g. Trichocereus pasacana (spines of 10-20cm in length).


Other cacti well noted in literature for having fierce or dangerous spines include most members of the genera Echinocactus and Ferocactus. These mostly barrel-shaped cacti have very stiff sharp spines that include ones with  recurved or hooked -spine tips. These spines with the hooked tips have been used historically as fishing hooks and others have a local reputation as ‘cow cripplers’, as the tangle of hooked spines have been known to cripple and kill livestock.


Ferocactus
Ferocactus
Ferocactus latispinus
Ferocactus latispinus


Apart from dangerous spines the reputation for the most irritating and painful of all cactus spines is deservedly for the genus Opuntia. Spines from this genus have microscopically barbed edges that also make it more painful and difficult to remove from flesh.
All Opuntias are also known to produce two types of these spines including very small ones called glochids that are produced in small clusters at the base of often larger spines. Glochids detach easily and can become airborne. They have been known to get into the eyes and mouth where they cause considerable discomfort or even excruciating pain.

Opuntia microdasys
Opuntia microdasys


On a more positive note about all these cacti spines, none are poisonous.
Edibility - All cacti fruit are edible. A few cacti plants have other edible parts but a word of caution here; the correct preparation e.g. leaching and/or cooking are sometimes the only things that make them safe to eat.

Opuntia robusta
Opuntia robusta


Many are recognised as having hallucinogenic or narcotic properties and therefore can also be regarded as being poisonous.

Succulents
Once again starting with the most dangerous first, I think the genus Agave stands out here. Many species have long often stiff to rigid leaves with piercing needle-sharp tips at the end, that can cause serious injuries especially if you were to fall on them.
The margins of most Agave leaves also have thorns that are often inward curving to the plants centre, making escape a delicate trick of unhooking oneself, while also watching not to get inadvertently hooked up by another leaf (as is often the case).

Agave parrasana
Agave parrasana


Aloes, mostly from Africa, are in many ways superficially comparable in appearance to Agaves, especially when not in flower. Many aloes appear as thorny but are much safer to handle and grow in a garden with children or pets around. Very few Aloes species are armed with dangerous or sharp thorns and almost certainly not able to kill or injury anybody in a serious way. Among the thorniest Aloes are Aloe ferox, A. marlothii and A.melanocantha, however they are more likely to only scratch or create superficial lacerations. The majority of garden Aloes have relatively soft, short thorns.

Now onto the subject of poisonous succulent plants (apart from cacti). Most gardeners or collectors of plants of any kind, do not like to find out their favourite plants can be dangerous or poisonous. Over-awareness of this can be off-putting, especially for visitors who may need to be cautioned on entry to your garden. All plants have evolved to survive some level of being eaten by man, animal or insect.  Two of the main ways most plants have evolved to deter this is by tasting bitter, being poisonous or both.

Many succulent plants have poisonous parts and there are far too many to list or explain in this article; as there are as many again that even scientists haven’t studied enough to give us an answer either way. 

Don’t try to eat or use them without the best expert, I repeat expert, advice. This does not include your neighbours and relatives, who can inadvertently pass on incorrect folklore and hearsay, no matter how much you know or trust them. Be cautious of all succulent plants, handling them with care and washing your hands afterwards.

Importantly though, there is one genera - Euphorbia, that has thousands of species, all of which bleed a milky white sap that is very corrosive and / or poisonous. Greater caution and preparation is required for handling all members of this large genus. Even a small scratch on the leaves or stems can start the sap flowing and often freely dripping. Wear protective glasses and gloves, as the sap can splash and has been known to cause blindness.
Also the vapour emanating from Euphorbia sap can irritate eyes and the respiratory system. Only prune or take cuttings in a well-ventilated, preferably outdoor area, with you positioned upwind..

Euphorbia candelabrum
Euphorbia candelabrum


Identifying a Euphorbia without really knowing them can be tricky, but is the first and most important step in responsible due care.
When not in flower or fruit, some look like cacti and others look like common perennial shrubs in the garden. If you think you’ve found one, the simplest check is to carefully scratch or pin-prick any part of the plant to see if it has the milky white sap, keeping well back after doing this - then you may have found one.
Being careful about any plant, including weeds, that bleed milky sap, is just as worthwhile even if it’s not a Euphorbia e.g. thistles and milkweeds from the Asclepiad family which can also be poisonous.

Euphorbia bupleurifolia
Euphorbia bupleurifolia


Euphorbias have a remarkable long history, mostly related to the plant’s sap, both as a healing agent and as a poison, and there is a wealth of literature about these in books and on the internet.

Euphorbia decaryi var. spirosticha
Euphorbia decaryi var. spirosticha


In summary there are many succulent plants, even apart from Euphorbias that are known around the world, or are considered in some way to be poisonous. While the internet and the information sourced from it needs to be verified or interpreted with caution, there are some very interesting sites dealing with this topic in more specific detail including ‘The Amateurs’ Digest’ that has a nineteen page document listing a great many poisonous succulent plants and much of what is written about each plant will actually scare people.  Be careful not to be discouraged growing these plants when researching them, because they are no more harmful than plants in our house and garden already e.g. daffodils, green potatoes, and rhubarb leaves which can be as poisonous.