Tales from the Medicine Chest

Aloin -

'Irritant laxatives, of which the anthraquinones contained in the yellow sap of Aloe are examples, produce laxation by poisoning the enzyme systems in the intestinal wall which are responsible for the absorption of water and nutrients. Chronic laxative use (laxative abuse) consists of chronic self-poisoning. It really cannot be recommended!'

Belladonna -

The name (a.k.a. Deadly Nightshade) belladonna originates from the historic use by women (Bella Donna is Italian for beautiful lady) to dilate their pupils; an extract of belladonna was used as eye drops as part of their makeup preparations. The Belladonna toxin's atropine content had the effect of dilating the pupil, thus making their eyes supposedly more attractive. Dilated pupils are considered more attractive (especially with females) because pupils normally dilate when a person is aroused, thus making eye contact much more intense than it already is. It had the adverse effect of making their vision a little blurry and making their heart rates increase. Prolonged usage was reputed to cause blindness.

The plant is an important source of atropine, ... Atropine will also reverse the effects of poisoning by nerve agents designed for chemical warfare. In Europe, the plant is specifically cultivated for this purpose....

Occasionally, the plant is used for recreational purposes: it is consumed in the form of either a tea or simply raw, which can produce vivid hallucinations, described by many as a 'living dream'. Upon consumption of this plant, the user will experience all the severe, adverse anticholinergic effects before hallucinating and continue to do so while hallucinating.

Strychnine -

used as a pesticide, particularly for killing small vertebrates such as rodents. Strychnine causes muscular convulsions and eventually asphyxia or sheer exhaustion. The most common source is from the seeds of the Strychnos nux vomica tree. Strychnine is one of the most bitter substances in the world. Its taste is detectable in concentrations as low as 1 ppm.
...
There is a common but false urban legend that strychnine is added to drugs like LSD or that strychnine is present in the peyote cactus. The dose of LSD is so small that it could not be mixed with a toxic amount of strychnine, even if strychnine made up an entire blotter square.
...
In the 1904 Olympics, Thomas Hicks (USA) won the marathon at St. Louis and collapsed. It took hours to revive him; he had taken brandy mixed with strychnine to help him win his gold medal.

Strychnine is the title of a song by 60s american garage rock band The Sonics (later covered by The Cramps), which includes the lines:
Some folks like the water
Some folks like the wine
I like the taste
Of straight Strychnine

In the movie Office Space, when Milton is at the resort before the credits roll, he is brushed off by a waiter. After muttering a few threats, he says, "I could put strychnine in the guacamole..."

It was also used by Norman Bates (PSYCHO) to kill his mother and her lover (as exposed in PSYCHO IV: The Beginning)

A tonic laced with arsenic and strychnine and religiously given to legendary racehorse Phar Lap may have caused his death.

Cascara -

a stimulating laxative.

Jesus ... what were these people thinking.

Eat your vegetables.

my bold