People (often times unknowingly) consume those items on a regular basis, yet they live. Arsenic was once used in medical applications, cyanide is produced naturally by foods such as bean sprouts, cashews, cherries, corn, lentils, potatoes and soybeans. Mercury is used in medicines that provided cures for syphilis, and is used in vaccines. Some organic halides are essential for proper bodily functions. Chlorides help in certain brain functions and helps to make stomach acids.
I dont recommend ingesting poisons in any amounts, and I thought it would be obvious that I didn't intend to include deadly substances. There are many claims that marijuana contains poisons, so smoking is basically poisoning oneself.