The erythromycin breath test (ERMBT) is a method used to measure metabolism (oxidation and elimination from the system) by a part of the cytochrome P450 system. Erythromycin produces 14CO2, and this 14CO2 can be measured to study drugs that interact with the cytochrome P450 system. Erythromycin is tagged with carbon-14 and given as an intravenous injection; after 20 minutes the subject blows up a balloon and the carbon dioxide exhaled that is tagged with carbon-14 shows the activity of the CYP3A4 isoenzyme on the erythromycin. ERMBT can be used to determine how drugs that the CYP3A4 isoenzyme metabolizes will function in a given individual. The ERMBT has been widely used and verified as a reliable method for measuring CYP3A activity in real time. Its non-invasive nature makes it useful for researching drug interactions and personalizing medicine dose for drugs metabolized by CYP3A. However, its results can vary in different clinical circumstances, emphasizing the difficulty of properly predicting drug metabolism and release.