> isotopes should be chemically identical. Not really. I don't know about lithium isotopes, but there are drugs "improved" by replacing some hydrogen atoms wit

> isotopes should be chemically identical. Not really. I don't know about lithium isotopes, but there are drugs "improved" by replacing some hydrogen atoms with deuterium (an isotope of hydrogen): https://en.wikipedia.org/wiki/Deuterated_drug https://en.wikipedia.org/wiki/Deuterated_drug Apparently isotopic food is also a thing: https://en.wikipedia.org/wiki/Ifood_(isotopic_food) https://en.wikipedia.org/wiki/Ifood_(isotopic_food) Edit: I found this 1986 reference about lithium https://www.ncbi.nlm.nih.gov/pubmed/3530801 https://www.ncbi.nlm.nih.gov/pubmed/3530801 Stable isotopes of lithium: dissimilar biochemical and behavioral effects. Lithium, which is used routinely in the treatment of mania, is composed of two stable isotopes, lithium-7 (92.58%) and lithium-6 (7.42%). Usually there is minimal physiological or biochemical differentiation between isotopes of an element, but lithium is an exception. Data derived from a variety of biochemical and behavioral experiments are reviewed to support this idea. Additionally, the clinical implications of this work are presented.