Cocaine is the most potent stimulant of natural origin. "Crack" is the smokeable free-base form of cocaine. These drugs stimulate the central nervous system and are extremely addictive. They can cause psychological and physical dependency. Stimulants can lead to dilated pupils, increased pulse rate, elevated blood pressure, insomnia, loss of appetite, paranoia, and seizures. They can also cause death by disrupting the brain's control of the heart and respiration. The use of amphetamines and other stimulants can have the same effect as cocaine and cause increased heart rates and blood pressure that can result in a stroke or heart failure. Side effects include dizziness, sleeplessness, and anxiety. They can also lead to hallucinations, paranoia, psychosis, and even a physical collapse. Nicotine is a highly addictive stimulant, whether ingested by smoking or chewing. This drug affects the brain in six seconds and damages the lungs, decreases heart strength, and is associated with many types of cancers when ingested by smoking. The withdrawal symptoms include anxiety, progressive restlessness, irritability, and sleep disturbances.