Folic acid is also called Vitamin B9. It is water soluble and present in food. It does not cause any false negative or false positive effects on the drug test.
Can Folic Acid Make You Fail Drug Test?
I would say there are some variables here you should mention. What are they testing for and how are they testing it. Vitamins are Amines (Vital Amines = vitamins) and wet tests are not very definitive. If your test is for opiates or meth those are amines as well. I would ask for a retest if you get a +. If the tests are done by GCMS they will likely be accurate but the tests in general should be Quantitative, not Qualitative.