Tasimelteon-d5 is intended for use as an internal standard for the quantification of tasimelteon by GC- or LC-MS. Tasimelteon is a melatonin (MT) receptor agonist. It selectively binds MT1 and MT2 receptors over a panel of 160 additional receptors and enzymes at 10 µM. Tasimelteon inhibits forskolin-induced cAMP accumulation with EC50 values of 0.79 and 1 nM in NIH3T3 cells expressing the MT1 or MT2 receptor, respectively. Formulations containing tasimelteon have been used in the treatment of non-24-hour sleep-wake disorder.