This depends on what you mean by worst. Some drugs, like tobacco, can damage you slowly – for example, long-term tobacco use can cause cancer and is the leading cause of preventable death. Many drugs, and alcohol, can impair your coordination and lead to devastating accidents (like drunk driving). Other drugs, like meth, can be especially toxic to your brain. Taking drugs, like heroin or cocaine, or misusing prescription drugs, has the potential to cause overdose and death even the first time you use them. They can also result in addiction. Any drug you use can be dangerous for you. Please protect your brain and your health—you need them! Check out the overdose death statistics for various drugs: http://www.drugabuse.gov/related-topics/trends-statistics/overdose-death-rates.