One of the ingredients in tobacco is a mood-altering drug called nicotine. Nicotine reaches your brain in mere seconds. It’s a central nervous system stimulant, so it makes you feel more energized for a little while. As that effect subsides, you feel tired and crave more. Nicotine is habit forming. Smoking increases risk of macular degeneration, cataracts, and poor eyesight. It can also weaken your sense of taste and sense of smell, so food may become less enjoyable.