Crack is a form of cocaine. Crack cocaine got its name from the crackling noise it makes while being smoked. It is highly addictive. It has been processed into rocks for smoking. Crack is the street name given to cocaine which has been processed from powdered cocaine into a smokable form. Formally known as freebase, crack cocaine is cocaine that has been processed with ammonia or sodium bicarbonate (baking soda) and water, and heated to remove the hydrochloride. Because crack is smoked, the user experiences a high instantly. In general, the faster the absorption of any drug, the more intense the high is. This rather immediate and euphoric effect is one of the reasons crack has become enormously popular in recent times. Another reason is that crack is less expensive both to produce and to buy.

    Some users of crack cocaine report feelings of restlessness, irritability, and anxiety. A tolerance to the "high" may develop many addicts report that they seek but fail to achieve as much pleasure as they did from their first exposure. Some users will increase their doses to intensify and prolong the euphoric effects. Crack cocaine use can result in a period of full-blown paranoid psychosis, in which the user loses touch with reality and experiences auditory hallucinations. Other complications associated with crack cocaine use include disturbances in hearth rhythm and heart attacks, chest pain and respiratory failure, strokes, seizures and headaches, and gastrointestinal complications such as abdominal pain and nausea. Because crack cocaine has a tendency to decrease appetite, many chronic users can become malnourished.

(Taken from: http://www.addict-help.com)

Why the users of crack doubled their dose?