Ibuprofen, loratadine, naproxen, and several other common over-the-counter medications are known inhibitors of glucuronosyl transferase, which are responsible for the metabolism and excretion of phthalates, ubiquitous industrial chemicals and environmental pollutants. Existing studies on phthalate diesters and their monoester metabolite concentrations in humans suggest individual metabolism of phthalates varies both between and within individuals, but the mechanism of this inter- and intra-individual variation remains unknown. Glucuronosyl transferase inhibition by over-the-counter medications may affect individual phthalate metabolism, and therefore may explain patterns in inter- and intra-individual metabolic variation. In the following study, novel liquid chromatography tandem mass spectrometry and solid phase extraction methods were developed to quantify over-the-counter medication levels in human urine of known phthalate monoester levels to assess their role in the inhibition of phthalate metabolism.