I wouldn't recommend threatening legal action, it's not America where legal action is an immediate retort.  Do demand an apology though, she's accused you of doing an illegal substance at work. However, go to your HR dept and inform them that one of your colleagues is trying to slander you.  If they do nothing (HR are generally fat, useless cunts imo) at least there will be a record that you went there.  So if Ham1 tries to pull a similar stunt in future there will be a trail.