Metabolism

How large a variation is there actually in human metabolism?

Variations can be anywhere from from less than an inch to 7 kilometers or more in size, depending on the compound being metabolized.

The liver cytochrome P450 enzymes involved in drug metabolism are notoriously variable from person to person.
This makes life difficult for pharmaceutical developers:
Implications of Polymorphic Cytochrome P450-Dependent Drug Metabolism for Drug Development