One such story was the sad case of Michael Marin who had been convicted in court of felony arson. There's convincing video evidence that after his guilty verdict, he appears to, not once but twice, swallow something and in less than 10 minutes is convulsing and is later pronounced dead.
A later news story reported that the substance Marin ingested was cyanide. But how does cyanide kill so quickly?
Cyanide works to lock up cellular respiration by inhibiting the cells ability to use oxygen. We think of respiration as the person breathing in and out. Well, there is respiration on a cellular level-- not the mechanical breathing in and out but enzymatic reactions that keep the larger host alive. Cyanide works at the cellular level to stop the energy cycle from working. If you don't have cellular energy-- you body's furnace burning to keep you alive-- you'll cease working as an organism pretty quickly.
Here're a few links that might explain cyanides effects on the body more eloquently than me.
There is an antidote for cyanide poisoning but it must be administered rapidly. There is a "Cyanide Antidote Kit" that contains several medications to try and get cellular respiration back on track. You can read about that here.
Have you ever used cyanide in a story line?