Answer
Yes. A guaifenesin metabolite can interfere with urinary 5-HIAA and VMA measurements, so tell the laboratory or clinician it has been taken.
Sources for this answer
- Benylin Mucus Cough plus Decongestant Syrup — Current selected SmPCemc · Summary of Product Characteristics
General medicine information. Follow the instructions for your own medicine and speak to a pharmacist or clinician about your individual circumstances.