To convert International Units (IU) of nystatin to milligrams (mg), you need to know the specific activity of nystatin. The specific activity of nystatin is typically expressed in IU per milligram.

For example, if the specific activity of nystatin is 2000 IU/mg, then 100000 IU of nystatin would be equivalent to:

100000 IU / 2000 IU/mg = 50 mg

Therefore, 100000 IU of nystatin equals 50 milligrams.