False positive drug testing?

My wife has recently had to take a drug screen for a job she just was hired for. She has taken 3 tests now and have came back positive for methamphetamines. She does not do any street drugs and never has, but this company has made her feel as if she is a criminal. She Does take perscribed medicines for her doctor for cronic back pain, migrains, and insomnia. She takes Hydrocodone which includes tylenol with codiene for her back. Tramdol for her back. Trazadone and hydroxyzine for her sleep. And topamax for her migrains. Those are all perscribed from her doctor. She has also had broncitis for the past 2 months and has been taking nyquil/dayquil, and mucinix. The drug test cup was the one you urinate in and pull the strip off the side and the results show on the side, which we have been hearing are cheap drug tests. This employer has refused to send her to the hospital to draw blood or even a urine test through the hospital because of price. Im just curious if anyone has any info on this and if they have knowlege of what may cause a false postive on these tests. Thanks