Somewhat related, but I attempted cyanide poisoning before; I grinded bitter almonds, mixed them with water and a lot of sugar then drank it all.
Bitter almonds contain amygdalin which turns into hydrogen cyanide when ingested. I felt no pain as my body grew number by the minute until I lost consciousness. Unfortunately I woke up later on with the urge to throw up. Apparently glucose acts as an effective counteragent to cyanide which thwarted my plan.
After that, I had extremely low blood pressure for 3 days straight and could barely stand up.