Primidone-d5 is primarily used as an internal standard in analytical chemistry, especially in mass spectrometry and liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods. Its main application lies in the accurate quantification of primidone and its metabolites in biological samples such as plasma, serum, or urine. Due to its deuterated structure, it exhibits similar chemical behavior to non-labeled primidone but can be distinguished by mass detection instruments, allowing for precise measurement with minimal interference from matrix effects. This makes it invaluable in therapeutic drug monitoring, pharmacokinetic studies, and forensic toxicology where reliable and reproducible results are critical.