I was falsely accused of taking drugs at work, passed a urine test the next day, and am still dealing with these false accusations. What must I do?

I was accused by a coworker of smoking marijuana on company property and attempting to convince the accuser to partake in the illegal activity. The following day I was asked to take a itinerary test, which returned negative results. Four days later I am still having to defend myself against this personal attack against my character. I need to know what steps I need to take next to protect myself against this person who is undeniably trying to get me fired.