(4R)-4,6-Dihydroxy-2-methyl-1-(3-methoxy-4-hydroxyphenyl)-1,2,3,4-tetrahydroisoquinoline-d3 is labelled (4R)-4,6-Dihydroxy-2-methyl-1-(3-methoxy-4-hydroxyphenyl)-1,2,3,4-tetrahydroisoquinoline which is an impurity of (R)-Phenylephrine Hydrochloride (P320640), an α-Adrenergic agonist.