Counterfeit medicines are an increasing worldwide problem with substantial economic consequences, and the World Health Organization estimates that up to 10% of drugs in low- and mid-income countries are counterfeits. Counterfeit medicines yearly cause thousands of deaths resulting from administering these poor-quality products. The European Union has a solid legal framework for the licensing, manufacturing, and distribution of medicines, and only licensed pharmacies and approved retailers are allowed to offer medicines for sale. Despite careful and restrictive regulatory control, cases of counterfeit medicines are increasingly reported in European countries. Identifying non-visual counterfeit medicines is challenging, and novel analytical methods are needed. Stable isotope ratio mass spectrometry (IRMS) analysis is a promising high-precision technique that has been an essential method for decades across different scientific fields. However, IRMS has never really gained momentum in pharmaceutical science.

This study investigated IRMS as a characterization technique for pharmaceutical drug products and commonly used excipients. Ibuprofen was used as a model drug for the IRMS method establishment, and the ibuprofen products were selected to demonstrate different formulation principles. Plant-based excipients followed expected C3/C4 plant 𝛿13C values, whereas 𝛿13C values of synthetic excipients were reported for the first time. The ibuprofen products demonstrated unique 𝛿13C values, where the observed differences could be interlinked with formulation strategies and choice of excipients. IRMS demonstrates excellent potential as a fast characterization technique for drug products and excipients for the quality assessment of raw materials and authentication. Additional studies are to expand the analysis to multidimensional light isotopic signatures, which can become an essential application for identifying and understanding non-visual counterfeit medicines.