I have a bottle of a nasal spray that I'm using for allergies, and it says on the label that each spray delivers "50 mcg" of the drug. What in the world is a "mcg"? Do you think they mean μg (microgram), which is 10-6grams, or in SI units a single spray would be delivering 5*10-8kg of drug, or do you think they mean m-c-g (milli-centi-grams), which would be 10-3*10-2grams =10-5grams, or in SI units a single spray would be delivering 5*10-7kg? These are two different values depending upon how I read the label, one of which is the actual amount of drug delivered and the other of which is either 10 times or one tenth the amount of drug delivered.
This is the whole reason we have metric prefixes in the first place, to reduce confusion and have a standard system whose meaning everyone agrees to. Good thing the actual quantity of the drug here doesn't matter to me, I just take my prescribed two sprays.