The international unit, or IU, is a frequent source of confusion for patients and clinicians alike. It doesn’t have an absolute value, but rather a relative one. For example, a kilogram is a measure of a specific amount of something. A kilogram of feathers and a kilogram of gold weigh the same. International units aim to allow comparisons of the biological effects among different forms or preparations of a compound.
To define the IU for a substance, the World Health Organization (WHO) Expert Committee on Biological Standardization does a study and sets a certain level of biological activity for one form of the substance as equal to 1 international unit. We can then compare biological activity between other forms using the IU as a reference point.
Since I suspect you’re asking about IUs because Tazorac is a retinoid, let’s use vitamin A as an example. There are two forms of vitamin A we can get in our diets: retinol, from eating animal food sources, and the carotenes, from plants. Your body converts it all to retinol eventually, but the carotenes require extra steps to turn it into retinol, so 1 milligram of beta-carotene has less “vitamin A effect” than 1 mg of retinol. This is where the IU comes in. For vitamin A, 1 IU is set as the activity found in 0.3 mcg retinol, 0.6 mcg beta-carotene, and 1.2 mcg other carotenes.
This is only the case for the select groups of compounds that the body absorbs through the digestive tract and convert to vitamin A. Tazorac (tazarotene) and the other retinoid drugs are not part of this group, and so it isn’t possible to specify IUs for them. Nor would it be useful, as these drugs only affect the skin area where you apply them, not the entire body activity that IUs indicate. To be clear, drugs like Tazorac do not provide vitamin A, and should not be considered as contributing to a patient's recommended daily intake of vitamin A.
As far as results go, Tazorac appears at least as effective as steroid-based topicals. For patients with plaque psoriasis, one study reported treatment success (greater than 75% improvement from baseline) in 45% of patients using the 0.05% strength, and approximately 63% for the 0.1% strength. Most patients will see improvement, but the degree of improvement and severity of side effects may vary a lot. Remember to advise patients using Tazorac to wear sunscreen if they'll be in the sun for more than 10 minutes, as all retinoid drugs make the skin extra vulnerable to UV damage.
Vectical is a topical ointment formulation of calcitriol, also called vitamin D3. This is the active form of vitamin D, the same form the body produces when the skin is exposed to UV-B light from the sun. The available strength is 0.0003%, or 3 micrograms per gram of ointment.
Unlike Tazorac, Vectical is absorbed in small amounts through the skin into the body, where acts like vitamin D normally would: increasing absorption of calcium from the intestines and decreasing the amount of calcium lost in urine. This can cause high blood calcium levels (hypercalcemia), which in one study was observed in 24% of patients receiving Vectical vs. 16% receiving placebo. The total elevation in blood calcium levels, however, was less than 10% of the upper limit of normal. Caution should be used in patients also taking thiazide-diuretics, calcium or vitamin D supplements (prescription or OTC),
Vitamin D IUs are set at 40 IU = 1 microgram, but because only a small
portion of the applied dose is absorbed into the body, IUs may be misleading or confusing for a product that is not intended as a vitamin supplement. However, the conversion would be 3 mcg/g = 120 IU/g.