Good question, and the answer will depend a lot on your philosophy in life and your purpose with your chickens (but unfortunately this will be settled beyond your control shortly by upcoming new government sanctions.)
The bottom line has to do with FDA approval and growing public concern over the safety of the food chain. There has been a lot of outcry from those concerned about too many antibiotics and synthetics in the food we eat causing harm and illness to humans.
The new FDA 2017 rules seek to restrict most meds for food animals to veterinary prescription only. The goal is by September 2017, but I have already seen a number of popular products drop off the feed store shelves in anticipation of the phase out.
The concern is the residue left in the animal from the med (in particular antibiotics) so that it cannot be passed to a human eating it. That's where studies were done to test how much of a particular drug was left in the animal after so many days. For some drugs a certain amount is/was allowed (new 2017 rules remember are coming), for others zero tolerance is/was allowed. There are a number of meds that are safe for one species but very toxic to another. (Ivermectin for example won't hurt birds generally but is quite toxic to kittens and collies, shelties, and a few other dog breeds due to a genetic mutation that allows drugs to cross the brain/blood barrier).
That is of concern if you sell eggs or meat to anyone. If someone should get sick after eating your eggs or meat and it was tested and found that it had a forbidden substance, you would face government sanctions, likely a nasty law suit, and financial actions.
However, if you simply raise for your own family use, then you have a little more latitude (except that congress and the FDA has removed a lot of the choices available with these new regs due to concerns also about over medicating food animals which has caused antibiotic resistant strains that have infected humans).
So let's assume you still have it on your shelf or can still get it and it works well on your chickens.
So my personal approach is to look at the med. Is/was it ever used for humans safely? Example, Ivermectin is, all over the world.
If so, then we can look at dosage and discover that the tiny amount for the chicken would not even come close to the dosage for a grown adult person who is not allergic to that particular drug.
I then don't worry about it or simply follow a few days pull time to avoid the worse of the concentration that might occur.
However some drugs are more potent and not used for humans. I'm more cautious.
Do keep in mind that some people can be frightfully allergic to some drugs, even in trace amounts (rare but happens).
However, if it is for your own birds and your own family use, then you can make the decision of whether you feel safe using that med.
Hope that helps.
LofMc