Irillium is an element which may occur as an impurity in ryetalyn. The presence of irillium in ryetalyn renders it pharmacologically inert, and useless as a treatment for Rigelian fever. (TOS: "Requiem for Methuselah")

Page 18 of the Star Fleet Medical Reference Manual describes irillium as having the chemical symbol Im, an atomic mass of 305 for its most stable isotope, and an atomic number of 116. It adds that the element (which is presumably radioactive) was first discovered by Humans in the Sol system's asteroid belt in 2029.

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