I've been taking 40mg Oxycontin every 12 hours for 1 1/2 years and Vicodin for chronic pain and had a negative urinalysis from last week. How is that possible and how often does that happen? I have had two accurate results in the last few months from the same place as the false-negative. This makes me and the Dr. look bad and the only thing that I can think of is my results were switched with someone else or they (Lab) didn't do it and said they did.