USP 41 requires using standard impurities F (2-methyl-1, 2, 3, 4-tetrahydroisoquinoline-4, 8-diol) and set a limit for the impurity 2-methyl-1, 2, 3, 4-tetrahydroisoquinoline-4, 6-diol (46THQ) for conducting the impurities test in phenylephrine products.