Heroin is a central nervous system depressant that relieves pain and induces sleep. It produces a dreamlike state of warmth and well-being. It may also cause constricted pupils, nausea, and respiratory depression, which in its extremes can result in death. Heroin activates brain regions that produce euphoric sensations and brain regions that produce physical dependence—hence its notorious ability to produce both psychological and physical addiction. Its addictiveness is characterized by persistent craving for the drug, tolerance (the need for larger and larger doses to get the same results), and painful and dangerous withdrawal. Withdrawal symptoms include panic, nausea, muscle cramps, chills, and insomnia. Heroin use during pregnancy increases the risk of miscarriage and stillbirth. Infants exposed to heroin in the womb go through withdrawal at birth and exhibit various developmental problems. Besides the danger of overdose, addicts are susceptible to malnutrition, hepatitis, pneumonia, and AIDS.