What could cause a meth false positive?

Today, I went to a Bradford rehabilitation center for a conference. In order to keep myself outpatient, I had to take a urinalysis.Now I’ve been clean for almost a month, so I was confident, but in the elevator my consultant informed me I had tested positive for meth. I informed them I was on a prescription codeine cough syrup, and said it also contained pseudoephedrine (I’m pretty sure). The clinician INSISTED that it wasn’t a false positive, and that my cough syrup, IN NO WAY, could have triggered it. I get a blood test tomorrow. But could there be any chance the clinician is wrong? I told them I wanted a more accurate test done, there is no way I’m gonna let a false positive set me back. PS: I also take Celexa 20mg and daily Yaz.

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