That's where things get a little more complicated. With compunds we need to see how the liver breaks them down and know what the final versions will be, Then its a matter of looking at the atomic weight of the compound. There are other factors too though from polarity (attraction/repulsion) to combined effects. Also one needs to consider mechanics in the organism from rate of flow to total load. This is where biouniqueness plays a major role and everyone will react differently.