Methoxy-13C labeled methacetin is primarily used in medical diagnostics, particularly in liver function tests. It serves as a substrate for the cytochrome P450 enzyme system, specifically CYP1A2, which is predominantly found in the liver. When administered, methacetin is metabolized into paracetamol and a labeled carbon dioxide (13CO2) byproduct. The rate of 13CO2 exhaled by the patient is measured using a breath analyzer, providing a non-invasive method to assess liver metabolic activity and function. This test is valuable in diagnosing liver diseases such as cirrhosis, hepatitis, and assessing the impact of liver damage from toxins or medications. Additionally, it is used in research to study drug metabolism and the effects of various substances on liver enzymes. The 13C labeling allows for precise tracking and quantification of metabolic processes.