That preprint is worth reading in full - the impurity finding is specific to mass-compounded tirzepatide and B12 combination products, not to plain compounded tirz or standalone B12 supplementation, so the first question is which formulation you're actually using. If you're buying a pre-mixed tirz and B12 vial from a telehealth-linked compounder, this is the scenario the study addresses. If you're reconstituting plain tirz and adding B12 separately, the picture is different. Worth knowing before assuming one way or the other.