Speaking in terms of how this relates to medicine, the recommended dosages of all medications are going to be determined on a case by case basis, meaning the dosage is specifically determined for each medication. Since you're only dealing with one medication at a time, each one has a dosage and an administration protocol associated with it, so it's actually quite straightforward.
Since you're usually dealing with extremely small quantities of a given drug, the metric system is preferable because it allows far greater precision when dealing with these tiny quantities. I'd rather have a medication expressed as ".05mg" than "1/516 oz". Far easier to deal with when dosage changes are called for as well. Frequently it's a merely a matter of moving the decimal and not much more.