Ok...so after many searches, this seems to be the most reasonable way....Take a small glass of cold tap water; do not use mineral water, nor any kind of juice or soda water, due to the acidity of such liquids.
Stir 1g (or 1.5g, at most) of potassium cyanide (KCN) into the water; using more than recommended will likely cause burning of the throat due to the acidity.
After about five minutesýthis "waiting period" is important, as a chemical reaction needs to take placeýthe KCN will be dissolved and ready to drink (because it has turned into HCN). It remains drinkable for a period of several hours, but not much more than that.
Once the concoction is drunk, consciousness will be lost within a minute. There will be just time to rinse out the glass (to ensure that no one else accidentally drinks from itýhowever, one could just as easily put a big "Warning" label onto the glass, or throw it into a corner or a fireplace, if one doesn't want to take the time) and lie down. But bewareýa person extremely weakened by illness might lose consciousness within twenty seconds.
While in the coma, death will follow in fifteen to forty-five minutes, depending on the physical strength of the person and whether the stomach is full or empty (an empty stomach promotes faster death).
During the coma period, the dying person will breathe heavily or snore, similarly to people who have taken a lethal dose of barbiturates.
Sometimes death is so peaceful that often doctors do not detect suicide, and sign the death certificate as being from natural causes.