Nicotine is an abundant poison. Easily found in tobacco products, in concentrated form a few drops can quickly kill someone.
Here is how to concentrate it:
First get a can of chewing tobacco or pipe tobacco. Remove the contents and soak in water overnight in a jar (about 2/3 cup of water will do…). In the morning, strain into another jar the mixture through a porous towel. Then wrap the towel around the ball of tobacco and squeeze it until all of the liquid is in the jar. Throw away the tobacco–you will not need it anymore.
Now you have two options. I recommend the first. It makes the nicotine more potent.
1) Allow to evaporate until a sticky syrup results in the jar. This is almost pure nicotine (hell, it is pure enough for sure!).
2) Heat over low flame until water is evaporated and a thick sticky syrup results (I don’t know how long it takes… shouldn’t take too long, though.).
Now all you have to do, when you wish to use it, is to put a few drops in a medicine dropper or equivalent, and slip about 4 or 5 drops into the victim’s coffee. Coffee is recommended since it will disguise the taste. Since nicotine is a drug, the victim should get quite a buzz before they turn their toes up to the daisies, so to speak.
Note: If the syrup is too sticky, dilute it with a few drops of water. And while you are at it, better add an extra drop to the coffee just to be sure!