Why Do Cigarettes Kill Brain Cells?

Of all the research I've done on this, I've never seen one piece of information able to prove that this happens.

Nicotine, and the chemical it breaks down to that actually affects the brain- cotinine- cause stimulation and a euphoric sense. High doses of these chemicals have an effect similar to cocaine. They are highly addictive, to be sure. But they don't cause actual damage to the brain.

The one case where I could imagine cigarettes killing brain cells is if you're smoking so much, and your lungs are so full of tar, that your ability to get oxygen to the brain is damaged and so brain cells begin to die because of that.

Getting oxygen to the brain is important. If you smoke, I think that your brain can be deprived oxygen. Nicotine is one way to do it, because it is addictive. If you only smoke once in a while that might be okay. But don't do it before an exam, you are killing the oxygen there for your brain to think. You won't remember your answers on the test. Duh! Stay away from Cocaine, your will be in a dream world, and will definately fail your test. And once you try it, it is suddenly addictive. One pill, your life is over.