Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval.

Taking MAO inhibitors with this medication may cause a serious (possibly fatal) drug interaction. Avoid taking MAO inhibitors (isocarboxazid, linezolid, metaxalone, methylene blue, moclobemide, phenelzine, procarbazine, rasagiline, safinamide, selegiline, tranylcypromine) during treatment with this medication. Most MAO inhibitors should also not be taken for two weeks before treatment with this medication. Ask your doctor when to start or stop taking this medication.

Guaifenesin and phenylephrine are available in both prescription and nonprescription products. Check the labels of all your medications to make sure you are not taking more than one product containing guaifenesin or phenylephrine.

Guaifenesin may interfere with certain lab tests (such as urine levels of certain acids), possibly causing false test results. Make sure lab personnel and all your doctors know you use this drug.