 This is useful for radiopharmacy, where it is difficult to work with technetium – especially the 99m isotope used in medicine – due to its expense and short half-life.Very little is known about the toxicity of rhenium and its compounds because they are used in very small amounts.Soluble salts, such as the rhenium halides or perrhenates, could be hazardous due to elements other than rhenium or due to rhenium itself.