i can't say i understood a lot about the metabolising stuff but here's what i did get, if it's any help.
there is a therapeutic range of the concentration of the drug in your bloodstream, and it has to be between a particular set of two numbers for the drug to be effective. too high can lead to poisoning and bad side effects, too low and it won't work. for lithium its 0.4-0.8mmol/l. everyone metabolises drugs differently, so the same dose in two different people will result in different concentrations of the drug in your bloodstream. they have to do blood tests to check the level, if it's too low they'll increase your dose, too high and they'll decrease it. when you're in the therapeutic range you won't have to have bloods as often as the concentration should remain pretty stable.
hope this is what you were asking, if not, apologies.
x