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Question:
What's the International Unit (IU) in pharmacology? How much is 1 IU?
 alessandro brunelli

Answer:
Hi! it seems to me, Alessandro, that my last answer didn't satisfy
you.  OK! I'll try to be clearer...  When you deal with commom units,
let's say using the Metric System,you say 1 l of water, or 1 l  of
milk, or 1 l of oil, all 3 volumes will be the same even though their
weights will be different due to density difference...  The same applies
when you speak of 1 kg of sugar, 1 kg of salt or 1 kg of cheese...
But...when we speak of International Units (I U) in Pharmacology,
these units are determined based at the biological activities.
For ex. let's take some vitamins:
Vitamin A    1 IU all-trans retinol              0.300 micrograms
Vitamin D    1 IU Vitamin D crystalline          0.025 micrograms
Vitamin E    1 IU dl-alphatocopherol acetate(s.) 1 milligramm
The I U is analogous with the United States Pharmacopeia unit (USP)
that also is based on measured biological activities.   See?
The use of IU is mostly connected with nutrition and nutritional
daily needs or requirements for human species and also for animals
and all kind of living organisms, aiming to provide one adequate diet
that will include the needed amount of a substance, mineral,factor,
essencial to life. So, if a specified factor is suspected of being
essential for the growth or maintenance of an organism, a systematic
series of procedures at the biological level must be used to determine
its function, effects of deprivation, and quantitative requirements in
various organisms. The results will then be expressed in terms of I U
of these factors...For extention the IUs are used to characterize the
amount of substances that are lethal, etc.                Mabel