Here is an extract from other website (fringe.davesource.com/Fringe/Information/Suicide_FAQ.html#Cyanide)
Cyanide (HCN, KCN)
Dosage: 50 mg Hydrogen Cyanide gas, 200-300 mg Cyanide salts
Time: seconds for HC, minutes Cs (empty stomach) hours (full s)
Available: very difficult to get hold of
Certainty: very certain
Notes: It helps to have an empty stomach (since the salts react
with the stomach acids to form H.C.). A full stomach can delay death
for up to four hours with the salts. Antidotes to cyanide poisoning
exist, but they have serious side effects. What you can do, is instead
of taking the salts directly, drop 500mg or so into a strong acid,
and inhale the fumes. This will be pure Hydrogen Cyanide, and you
should die in 10 to 20 seconds.
[3]:
"Hydrocyanic acid is one of the most poisonous substances known; the
inhalation of its fumes in high concentration will cause almost
immediate death. Hydrogen cyanide acts by preventing the normal
process of tissue oxidation and paralyzing the respiratory center in
the brain. Most of the accidental cases are due to inhaling the fumes
during a fumigating process. In the pure state it kills with great
rapidity. Crystalline cyanides, such as potassium or sodium cyanide
are equally poisonous, since they interact with the hydrochloric acid
in the stomach to liberate hydrocyanic acid. This poison has been used
for both homicide and suicide; in recent history, a number of European
political figures carried vials of cyanide salt for emergency
self-destruction and some used them. Death resulted from amounts of
only a fraction of a gram. A concentration of 1 part in 500 of
hydrogen cyanide gas is fatal. Allowable working concentration in most
of the United States is 20 ppm. Two and one-half grains of liquid acid
has killed. The acid acts fatally in about 15 minutes. The cyanide
salts kill in several hours. The average dose of solution is 0.1 cc.
[1], DGHS talking about KCN]:
on an empty stomach, take a small glass of cold tap water. (Not
mineral water nor any sort of juice or soda water because of it's
acidity). Stir 1 -> 1.5 grammes of KCN into the water. More than that
causes irritation to the throat. Wait 5 minutes to dissolve. It should
be drunk within several hours. Consciousness will be lost in about a
minute. Death will follow 15 -> 45 minutes later.