Numerous Wrongful Accusations of Drug Use

I am employed at a privately owned mechanic's garage with 3 other mechanics. During the last few weeks, word has come back to me that two of my co-workers have been making remarks that I use drugs. They have made these remarks to other co-workers (one being my best friend), my employer, and the owner of another mechanic's shop. I used to smoke marijuana, but haven't for a long time. I discussed the problem with my employer, and he had a talk with one of the employees (stating he would fire him if the rumors didn't stop). The problem is still going on. One of the main problems, is that my checks are getting smaller and smaller. I am also under the understanding that one of the co-workers wants my job. If the rumors continue spredding throughout the shop and around town, I'm afraid they will ruin my reputation! HELP- I'm upset and just want it to stop, but I'm afraid I'll have to seek some type of legal assistance. Does anyone have any advice???