Nicotine

Nicotine makes receptors in your brain very active and your body doesn’t regulate the amount of nicotine that is released, so that many parts of your body could be stimulated at the same time. This stimulation of nicotine leads to an increase in the release of acetylcholine in the body, so there is an increased amount of cholinergic activity. This makes you feel more awake.
The stimulation of cholinergic neurons releases dopamine in the brain’s reward pathways. This neural circuitry causes you to do things that are necessary for survival, like eating. Having these necessities makes you feel good, so you are more likely to do it again.
Nicotine also triggers the release of glutamate, which enhances the connection between different sets of neurons and helps with learning and memory. So nicotine may remind you of previous experiences that you have had with nicotine and make you want to do it again.
In addition to these neurotransmitters, nicotine also causes your brain to make more endorphins in response, which may give you a sort of high.
Long term effects of using nicotine are risk of cancer, emphysema, heart disease, and stroke. Although one usually must consume a significant amount of tobacco in order for these effects to set in, people are advised to avoid smoking or chewing tobacco because the most dangerous thing about nicotine is its addictiveness.
Nicotine is an agonist, because it stimulates other chemicals rather than repressing them.
In order to better combat the effects of nicotine misuse, you could not exercise. This is because exercise produces endorphins, so you would have less if you did not exercise after misusing nicotine. You could also distract yourself from smoking in order to avoid a relapse. Some people use nicotine gum or patches. You could also do activities that you find pleasing, because this will naturally stimulate the release of glutamate.