Many drugs, whether prescribed or abused, affect the nervous system. While about 25% of all prescribed drugs are taken to alter psychological conditions, almost all the commonly abused drugs affect mood. In particular, levels of serotonin and dopamine are thought to influence mood. For example, when excessive amounts of norepinephrine are released, people feel energetic and stimulated, whereas low concentrations of this neurotransmitter reduce anxiety. Habitual use of almost any mood-altering drug can result in psychological dependence, in which the user becomes emotionally dependent on the drug. When deprived of it, the user craves the feeling of euphoria (well-being) that the drug induces. Some drugs induce tolerance after several weeks. This means that response to the drug decreases, and greater amounts are required to obtain the desired effect. Tolerance often occurs because the liver cells are stimulated to produce more of the enzymes that metabolize and inactivate the drug. Use of some of the drugs, such as heroin, tobacco, and alcohol, may also result in addiction (physical dependence), in which physiological changes occur that make the user dependent on the drug. Addiction can also occur because certain drugs, such as morphine, have components similar to substances that body cells normally manufacture on their own. The continued use of such a drug causes potentially dangerous physiological effects.