1. More crime. Rarely do people get high and then run out to commit crimes. Most "drug crime" happens because the product is illegal. Since drug sellers can't rely on the police to protect their property, they form gangs and arm themselves. Drug buyers steal to pay the high black market prices. The government says alcohol is as addictive as heroin, but no one is knocking over 7-Elevens to get Budweiser.
2. More terrorism. The profits of the drug trade fund terrorists from Afghanistan to Colombia. Our herbicide-spraying planes teach South American farmers to hate America.
3. Richer criminal gangs. Alcohol prohibition created Al Capone. The gangs drug prohibition is creating are even richer, probably rich enough to buy nuclear weapons. Osama bin Laden was funded partly by drug money.
Government's declaring drugs illegal doesn't mean people can't get them. It just creates a black market, where even nastier things happen. That's why I have come to think that although drug addiction is bad, the drug war is worse.