Marijuana is the most commonly used illicit drug in the United States. When smoked, it begins to effect users almost immediately and can last for one to three hours. Users claim that smoking marijuana is not harmful, but scientific evidence proves otherwise. The truth is even the "safest" drugs can have negative effects on your health and well-being if they are used too often or over too long a period of time. Besides alcohol (which has its own set of health effects), the following are the most commonly abused drugs in the United States, in order of popularity, and how they can effect your health.