Approved? Approved by who or what? There is no real authority over any of this stuff. The list will vary by country, by individual, by state, by opinion.... I routinely get contradicted on here by recommending things that someone with many many thousands of posts, (and maybe three or four years of chicken keeping), will tell me with great authority what I should and should not be using.... Just saying.
I understand your frustration at the obvious ambiguity and conflicting information.
As to this thread, "approved" for drugs used as meds and wormers is the FDA...Food and Drug Administration, of the US. (EPA could also be involved for environmental impact, but anything used for livestock for human consumption comes under FDA).
Some poultry owners keep those regs in mind as the FDA does scrutinize for residues in eggs and meat that may harm the humans that consume them. (Obviously those who prefer "organic" reject any drug/med use altogether, and are not affected other than recognizing what is considered a drug or med).
The difference between the original 2015 list posted by Kathy (now under construction) and the amended 2017 list I linked to is law having been passed by Congress which took effect January 2017. With more and more public concern over antibiotic misuse and overuse in livestock creating superbugs that infect humans, the antibiotic related meds used in livestock that are also used in humans have essentially been taken off the market altogether or are by veterinary prescription only.
Those that deal with external parasites are scrutinized for environmental impact as well as food chain impact. (How does it affect our environment, residues, etc.; how does it stay in the meat/eggs, etc.)
So it will boil down to what you can find in the US over the counter and what you will need a veterinary prescription for.
And drugs can go in and out of favor. Drugs also go off the approved list not because they are "banned" (found dangerous) but because the manufacturer no longer finds it profitable to undergo all the testing and application process to become FDA approved.
For the family owner who uses eggs and meat for personal use only, these regs only effect the drugs available over the counter.
For those of us who sell eggs, or even give them away to friends, it is important to know what is considered legal use in the United States.
Why? Because if someone were to get sick and blamed your eggs, and residue from an illegal drug were found in the eggs, then you are fully liable under FDA law and may receive full weight of FDA penalties.
There are no egg police for the little guy, so it will occur only if something has brought your eggs under scrutiny...typically food borne illness by a customer or friend. (And if you think my friend would never sue me, think again...it won't be your friend who sues you, it will be their health insurance company recuperating their losses and the medical personnel treating which will tip off the FDA authorities).
For those who sell larger scale such that they come under FDA regulations as a poultry operation (about 3,000 birds I believe in the US), their eggs are regularly inspected and screened by the FDA inspectors.
Different countries do indeed have differing opinions as to what is safe and what is not safe....which argues for the somewhat arbitrary opinions over the actual risk of the drugs themselves.
The FARAD article linked by Kathy shows the suggested residue withholding times for the backyard holder to avoid risking residue discovery in eggs....should someone get sick and your eggs were tested.
I personally, having been a paralegal by trade (now semi-retired as a home tutor), who does sell eggs to offset feed costs, am concerned with FDA status of anything I put into the hens that I sell eggs from. (Comes from years of watching other people get in trouble for stuff).
Technically, if you ever use an unapproved med in your hens, you are to never sell eggs or meat from that bird ever again to avoid residue discovery. Proper withholding times can also result in no residue, however, you cannot state your eggs/meat are organic.
Med use or non-use is a problem once you get larger flocks or even sustain a smaller flock on the same land for a long period. My daughter and her husband are organic farmers who must meet organic standards, which means no drugs whatsoever. It also means that animal care is a lot of hard work since 99% of the treatment is prevention...using herbals to curtail infestations and strict rotation of fields and flocks to prevent parasite and disease build up....then culling any unhealthy animal.
LofMc