Why does anxiety appear after smoking?

The brief positive emotional experience (ie, affect) accompanying smoking derives from the rapid neuromodulatory effects of nicotine in the glutamatergic and dopaminergic pathways that extend between the mesolimbic centers and the prefrontal cortex. This is the basis of addiction.

Regarding a bad mood. One of the versions is that smoking increases the amount of deoxygenated hemoglobin in the blood in the area of ​​the prefrontal cortex. Simply put, your brain is getting less oxygen than it should be. From there all the problems. The study authors also found that 30 minutes of vigorous physical activity after smoking can significantly reduce this harm. Smoking and exercising is still a perversion, but better than just smoking and sitting on your butt straight.

By the way, this is the latest research that came out just the other day, right on your question.

Monroe, Derek C., et al. "The effects of exercise on mood and prefrontal brain responses to emotional scenes in smokers." Physiology & behavior 213 (2020): 112721.

It is also believed that after the initial pleasant effect of smoking, after a while withdrawal syndrome occurs, i.e. craving for nicotine. Many people smoke to relieve stress, anxiety, but the paradox is that smoking often causes this anxiety or exacerbates the existing one. It turns out a vicious circle - you smoke to relieve stress, so it only increases, and what are you doing? right, you smoke more and make it worse. You feel good while nicotine acts for a short time, as soon as its effect disappears, the withdrawal syndrome begins. Anxiety is one of its symptoms.