There were at least two major unintended consequences of Prohibition. Both, not surprisingly, were bad.
First, and perhaps less importantly, Prohibition helped to reduce people’s respect for the law. Prohibition seemed to cause people to actually want to drink more. Going to places like speakeasies became cool and desirable. This meant that all sorts of people who were otherwise law-abiding were all of a sudden flouting the law. Breaking one law on a consistent basis can help cause people to have a more cavalier attitude towards the laws in general.
Second, and more importantly, Prohibition allowed organized crime to flourish. Even after Prohibition was imposed, there was a huge demand for alcohol and for places in which to drink it. Since making, transporting, and selling alcohol was illegal, it was up to criminals to meet this demand. Since the demand was so high, the criminal gangs became rich. They also became much more widespread.
In these ways, Prohibition had fairly serious unintended consequences that helped make America in some ways a more lawless place.
The main purpose of Prohibition had been to reduce alcohol use in the United States population. There were two major areas of concern, public health and social issues. Doctors felt that alcohol, especially when consumed in large quantity, could harm people's health. On a societal level, many people felt that alcohol contributed to crime, domestic violence, and the growth of "saloon" culture. While Prohibition did succeed in reducing alcohol consumption, it had two major unintended consequences.
First, it contributed greatly to the rise of organized crime. Also, because enforcement was at best spotty, many people flouted the law, leading to a cultural decline in respect for law as such. These criminal groups, with the repeal of Prohibition, moved into related businesses such as other forms of drug dealing, and thus one could argue that Prohibition helped create the infrastructure for the more recent drug cartels and problems of drug addiction.
Second, Prohibition led to a decline in tax revenue that the United States, could ill afford.