Should we terminate an admitted drug user?

Q. Following a recent accident on our loading dock, an employee admitted he used marijuana before his shift. Afterward, the employee and the ­others involved were required to submit to a for-cause drug test pursuant to our drug testing policy. The employee who admitted being high failed the drug test. However, there was an irregularity with our testing vendor, and it was not able to complete the confirmatory re-test of the employee’s specimen. Can we terminate the employee who admitted to working under the influence of marijuana?