Why is nicotine a toxin i wanted to use it as a nootropic why cant i it helps with reaction speed and memory ?

Nicotine is one of many drugs with central nervous stimulant effects. In moderation it can be tolerated by most. In those 15-25 years old it has a tendency to alter the frontal lobes of the brain at any dose, producing a high rate of addiction. In high enough doses it can be lethal. You may consider it a "natural" drug, but so is snake venom. Use if you must, but you will pay a price.