In 1920s America, the sale of alcohol was prohibited. This basically means that it was illegal to possess or sell it, in the same way as cannabis, cocaine and ecstasy are illegal as of 2010. There are some positive points and some negative points to prohibition of substances.
People can become addicted to many drugs, including alcohol and cocaine, and addiction can destroy people's lives. If you drink or take drugs everyday it can affect your personal and professional life for the worst. Alcohol also damages the liver and kidneys, and according to the Center for Disease Control and Prevention, alcohol-induced liver disease was responsible for 14,406 deaths in 2007.
Banning people from voluntarily ingesting a substance has issues. A drinker or a drug taker is only potentially harming himself, and many argue that people should be free to choose do that. Prohibition also encourages black market trade. If alcohol is banned, because so many people drink it, they would want to continue. The illegality would mean that people would be more likely to buy alcohol from criminals, and therefore fund criminal activity.
Prohibition could be beneficial for people's health and to reduce addiction, but attempting to restrict people's freedom causes problems. Also, current drug laws haven't got rid of illegal drugs, and 1920s prohibition gave rise to the Mafia, so history teaches that prohibition can have negative consequences.