Gallons and milligrammes are two different units of measurement and cannot be compared. In contrast, 1 gallon of water is typically equivalent to 3785.411784 milligrammes of the drug in issue if it is water.
A gallon is used to measure volume, while a milligramme is used to measure mass. One milligramme of water occupies precisely one cubic centimetre. One gallon is equal to 3785.411784 cubic centimetres, according to UnitConversion.org. However, not all chemicals offer this simple conversion.
There are other materials with various densities. A measure of density is the ratio of mass to volume. The previously stated conversion of water may not be precise depending on the height at which the measurement is made because air pressure might have an impact on density.