From: Better Thinking Through Chemistry

Smart drugs can basically do three different things to your brain: either minimise the damage to the brain and the natural deterioration of one's brain functions, repair some of the damage already done or enhance brain functions above usual levels. Human nervous system deteriorates with age through natural ageing process and sometimes due to drinking or smoking. This deterioration is usually caused by an oxidation process, which destroys brain cells and form free radicals that cause further havoc in one's brain. Even though brain cells likely cannot regenerate, it is possible to deactivate free radicals and repair some of the other damage (such as low levels of electric current transmitting chemicals in the brain). And this is can be achieved with smart drugs and some nutrients, such as vitamins.