Drug addiction is a chronic brain disease characterized by compulsive drug use despite harmful consequences. Signs of addiction include behavioral changes like missing important events, emotional changes like irritability and loss of interest, and physical changes like changes in energy levels and appearance. Drugs of abuse fall into three categories - depressants, stimulants, and hallucinogens - and are consumed in different ways. Drug use can damage health over time and cause issues like cognitive impairment, accidents, mood disorders, heart and lung problems, and even brain damage. Different drugs also disrupt normal sleep patterns.