Will phenylephrine hcl cause false for meth in hair follicol

Phenylephrine, pseudoephedrine, ephedrine, and decongestants common in many over-the-counter cold medicines are known to cross-react with the amphetamine assay. (Mayo Clinic)*

Hair tests are conducted in 2-basic steps; 1) immunoassay and 2) confirmatory testing. Many OTC substances, including phenylephrine can cause a false positive at step 1 but would be properly identified in step 2.

In the event a false positive is generated then the additional confirmatory test, step 2, is required. That does not occur if step 1 is negative.

So long as the test is properly and completely administered and even in the event of a step one false positive your final result would be reported as negative.

*article relates to urine screens but step 1 is the same regardless. For procedural description of hair testing see "Can hair be affected by cross-reacting substances."