Correlation does not prove causality. Those likely to do drugs on the job are just as likely to be the type of people who would miss work and perform poorly whether they were on drugs or not. Furthermore, drug use can actually help job performance by correcting things like anxiety and chronic fatigue where the War on Drugs has made obtaining adequate treatment through legitimate medical channels impossible due to the unwillingness of a lot of doctors to prescribe controlled substances, even where the need is real.