Masses and masses

There has been for some time a dispute, albeit a muted one, between those who prefer to report drug concentrations in units such as mg/l and those who prefer to use μmol/l (see, for example, J Clin Psychopharmacol 1999;19:1-2). Toxicologists, who need an excess of molecules, prefer mass units. Pharmacologists generally prefer molar units and need just enough molecules. Immunologists often talk in mass units, if only because they don't know the exact molecular weight of the relevant species, but they really only need only one molecule at a time. Homoeopathists, on the other hand, talk in dilutions, not units of any sort, and they need no molecules at all.

Some seem to think that μmol/l …