Nicotinic-D4 acid is primarily used in research and development as a labeled compound in metabolic studies. Its deuterium atoms make it a valuable tool for tracing and analyzing metabolic pathways, particularly in the study of vitamin B3 (niacin) metabolism. It is also employed in pharmacokinetic studies to understand the absorption, distribution, metabolism, and excretion of nicotinic acid in biological systems. Additionally, it serves as an internal standard in mass spectrometry for accurate quantification of nicotinic acid in various samples.