What Causes False Positive On Drug Screen When Took Only Percocet?

What could have caused me to have a false positive on a drug screen when I only took the percoset 10-325 I was prescribed plus vitamins that I have been taking for my b12 deficiency I got after having stomach stabling which is really not a minor thing. The only other medicine I took was for a cold the week before I went to the doctor. I took other the counter medicines that the Pharmacist had recommended. I also take Milk Thistle for my chronic hepatitis which I have had for 30 years. So what can I do to prove to my doctor that I did not do any other drugs, which is absolutely true since I can verily move until I get my knee replaced. This follows 2 hip replacements that got me on this medicine, which I was not familiar with the addiction rates. I had planned to try and either get rehab so I would not die when they take me off, but per her advice do not get knee replacement until it is warmer because so easy to slip on snow and we are now living in an igloo from all the snow, which I have fell several times. My doctor actually said that she had the hip surgery than the knee surgery, just like me, and than she fell and the doctor told her if she fell again, they would put a total leg cast on her. She told me, she has put her dog in the kennel till the weather as broke, which actually made me wait till Spring. Now she has sent a letter that I broke the contract and was removing me as her patient. I never go anywhere now because of my knee, what can I do. This is totally wrong and I know I will not survive if she just cuts me out cold turkey. I really had planned to jump on this with help, because I had actually not realized that they addicting and just quit taking them for a while, now looking back had been going into hospital and getting IV s of nausea medicine and morphine because pain was so great, and did not realize than that it was the not taking of my medication that had done. I was sick and just quit taking them and would go into hospital for full bottle of viccodins and they would give me medicine and I would go home. But looking back iit had to do with me quitting taking the pills when I felt under the weather. So what do I do not to end up not making though a quick withdrawal and not being able to walk to get someone else to care for me. My granddaughter now takes me once a month to doctor and than to store and than home which is where I always seem to stay. Help Help Help