Nicotine

For those who are not familiar with detective stories and toxicology, nicotine is an ideal drug to poison your adversary for several reasons:

First, nicotine is highly poisonous. For someone who never smokes, 60 mg of nicotine (the amount in about 30 to 40 cigarettes) is lethal - by weight, it is 100 times more toxic than cocaine.

Second, it could be absorbed by almost every means of administration - including topical. Spilling an extremely high concentration of nicotine onto the skin could cause intoxication or even death. (For that reason, the spiky weapon that killed Harley Longstreet was a silly device; the malicious stock broker could easily be wiped off by simply spraying the poisonous droplets on his skin.)

Finally, nicotine is readily available. There is no difficulty to get tobacco leaves, and, nicotine-based insecticides are still being widely used in the America and Canada - although their use is barred in Europe. For the latter, the concentration of nicotine is quite high to begin with, and you can get an enriched product by using a low-temperature distillation to evaporate off the (alcohol-based) solvent. Rumor says that if you slowly cook a large pan of tobacco leaves in ethanol (in a set up similar to how we melt chocolate chips in our kitchen), you can also get a highly concentrated nicotine extract.

Oh, by the way, don't try the last method at home. The alcohol vapor from the boiling pot is also full of nicotine, and you would probably poison everyone in your house before you could harm your adversary. 

And, more importantly, I don't really think it works.