4-[4-[4-(Hydroxydiphenylmethyl)-1-piperidinyl]-1-buten-1-yl]-α,α-dimethyl-benzeneacetic Acid-d6 is labelled 4-[4-[4-(Hydroxydiphenylmethyl)-1-piperidinyl]-1-buten-1-yl]-α,α-dimethyl-benzeneacetic Acid (H825430) which is an impurity of Fexofenadine (F322470), an active metabolite of Terfenadine (T114500). Terfenadine (T114500) is a H1-histamine receptor antagonist.