For example, improper use as a topical treatment against head lice ( shampoos ) selected resistant parasites and there are now strains of head lice that resist Ivermectin. Stopping over-the-counter sales wouldn't have prevented this because there was no way all patient would dutily follow all the relatively bothersome treatment guidelines to prevent re-infestation and lice exposition to insufficient doses.

Ironically, the irrational fear of giving Ivermectin as a pill to swallow that facilitated the rise of lice resistance to it. But again, some people wouldn't have followed proper oral treatment plan unless it was in the form of a single pill to swallow and be done.

As a pill to swallow over a sifficiently long period, any lice feeding on patient gets killed by a sufficient dose, evem if the person didn't properly eliminate all re-infestation sources from their life.

With a shampoo, effect is topical, short-lasting, and the patient can re-infest himself with clothes, pillows, couch, etc, exposing those lice to doses that won't be lethal to more resistant lice, which will give some offspring with slightly superior resistance.

Those offspring will spread to different people, some of which will repeat the failure of treatment. After a few generations you have some lice that won't be killed by the previously 100% effective shampoo with proper topical application.

It won't kill head all lice at the formerly 100% effective oral dose either now.

Same problem with improper use of pesticide in agriculture.

Counter-argument : prescription-only government control didn't stop any of this from happening with anti-parasitic medicine, anti-biotics, anti-virals, etc.

What prevents this from happening is finding a 100% effective treatment that is impossible to fuck-up, such as a single dose treatment. Which is rarely possible. So we're stuck with the second-best option : research to develop new treatments and discard those that no-longer work.

Oh and having combined treatments with two, three is better, different acting mechanism drastically reduces the chances of developping resistance. HIV anti-restrovirals are a combo of 3 different drugs with totally different acting mechanisms for this exact reason.