Additional information.
Your 15 milliliters is 15 milligrams of water, but there is no direct conversion for other substances because of their different densities. Milliliters is a measure of volume, and milligrams is a measure of mass (weight). This question is similar to the question, "How do you convert gallons to pounds?" A gallon of water weighs more than a gallon of gasoline, and way less than a gallon of liquid mercury. Do you see the problem? Unless you have a conversion factor for the substance or a density or specific gravity of that substance, you can't get there from here.