L-Carnitine, a naturally occurring quaternary ammonium compound, is extensively investigated in biomedical research for its essential role in mitochondrial energy metabolism, primarily by facilitating the transport of long-chain fatty acids across the inner mitochondrial membrane through enzymes like carnitine palmitoyltransferases, enabling β-oxidation and ATP production while modulating the acyl-CoA/CoA ratio to prevent cellular toxicity. Experimental studies highlight its applications in treating primary systemic carnitine deficiencies caused by genetic impairments in biosynthesis or utilization, as well as secondary deficiencies arising from inborn errors of metabolism (e.g., glutaric aciduria II, medium-chain acyl-CoA dehydrogenase deficiency), end-stage renal disease, and drug-induced toxicities such as valproic acid overdose, where it enhances hepatic urea synthesis and survival rates. Clinical research explores its therapeutic benefits in conditions like myocardial ischemia-reperfusion injury, hyperlipoproteinemias, Alzheimer’s disease, diabetes, heart failure, and HIV lipodystrophy, demonstrating potential in reducing lipid accumulation, supporting ketone metabolism, and counteracting mitochondrial disruptions. Ongoing investigations focus on its pharmacokinetics, including nonlinear clearance and renal reabsorption dynamics, interactions with hormones (e.g., as a peripheral thyroid antagonist), and roles in branched-chain amino acid catabolism, positioning L-carnitine as a versatile nutraceutical and pharmacological agent in metabolic and neurological disorders.