How they used to get rid of motor oil back in the day.

"Feed a cold, starve a fever." Rest, hydrate, and eat if you can.

Aristotle was obviously a great teacher and philosopher but he ended up being wrong about a lot. Like he thought the “elements” were earth, wind, fire, and water and that all objects want to be in their “natural” place. So, if you drop a rock, it tries to return to the earth. Fire goes up because it’s trying to get to where it “wants” to live.

He thought eels didn’t procreate because no one had ever seen it happening. (They go out to sea to fuck.) He was into bees and correctly noticed that there were workers and drones and that young bees grow out of the honeycomb. But he just assumed the Queen was a King and that worker bees were out collecting tiny baby bees from flowers. (He thought the air just blew pollen around and the honey naturally appeared.)

He had a lot of ideas that were just ideas but he was so influential and his writings were preserved and translated. It took a shocking number of years for people to question if Aristotle was full of shit.

Check out the history of bird migration science. There was everything from birds going to the moon for winter, swallows burrowing in the mud, transmorphing to different species, up to the 19th century

Lightning never strikes the same place twice. In fact it favors repeated strikes at the same arcing point.

In the middle ages churches would ring the steeple bells during a thunderstorm in an effort to soothe God. (it was assumed the Christian God was directly responsible for lightning.) This resulted in such an epidemic of lightning deaths among parish priests that ringing church bells in thunderstorms remains a criminal act in some regions of Europe.

Modern cathedrals and statues are fitted with replaceable lightning rods, in an admission God is content to let the mechanics of static electricity guide His thunderbolts.

Classic case of survivorship bias

People back in the day had just as much terrible advice as we have today, it's just that the only one that survived long enough to survive to the present day is the really good advice

But to answer the question, anything related to the ingestion of mercury

Not even ancient- we used to prescribe cigarettes to cure asthma.

witch burning

I read Montaigne's essays (written in the 1500's) and while his views are remarkably modern in many ways, one thing that stuck out to me was how unabashedly elitist he is. The translation I had used the phrase "common herd" to refer to the large majority of people who failed to impress him due to their lack of education or strength of character. I hesitate to speak for him since I think he was a wiser man than I am, but I expect that our modern notions about democracy would have seemed ridiculous to him. He might accept that universal suffrage is in practice the least-bad option currently available to us, but he would argue that at least in principle it would be better to exclude people who don't actually know how to run a country from the process of deciding how the country is to be run.

(He would also be unashamed to say that the life of an exceptional person is worth more than the life of someone ordinary, but we think that in the modern day too. We just consider it rude to be so explicit about it.)

Its perfectly safe to burn any and all trash

Especially batteries

Most forms of medical advice, some of it stuck around for a long ass time (bloodletting and the idea of spirits and humors lasted several millennia), but I imagine that the vast majority of it is lost to time.

You don't even have to go all that far back to see this in action.

In the 90's, the universal medical advice was to avoid fats, sauces and dear lord never eat more than 2-3 eggs in a week or you'll have a coronary before 40.

You still shouldn't go overboard with fats and sauce which is made with fat, but the advice that you shouldn't eat more than 2-3 eggs in a week is entirely defunct now.

You can eat 2-3 eggs a day (which many people do without even knowing as eggs are used in a whole lot of things) without any medical disadvantages.

Read the theories of René Descartes (17th century) about the nature of air and the atmosphere. Try to get his original texts (translation if needed), not any secondary works.

It is some seriously sick stuff, from today's point of view :-)

At his time he was quite a renowned scientist.

Phlogiston, its where heat comes from!

https://en.m.wikipedia.org/wiki/Phlogiston_theory

Babylonian astronomy

https://en.m.wikipedia.org/wiki/Babylonian_astronomy

The Ether/Aether

That there is an invisible structure all around us that allows gravity, light and electricity to move through it. Now debunked or replaced.

Trepanning to release evil spirits.

Drill a hole in your head as a cureall for any mental behaviour abnormalities. Still practised as an emergency surgery, only to release life-threatening blood and pressure buildup inside the cranial cavity.

Blowing smoke up your ass

Gut pain? Almost drowned? Time to blow some tobacco smoke up your bum. Discontinued.

HEAD ON APPLY DIRECTLY TO THE FOREHEAD!

Drowning women to determine if they are witches.

Medicine and not taking anything as the will of god you should just accept, this and perception of death. That direct war, colonies are necessary - because now soft power, investments, influence, proxies are seen as more effective and better for business. That raw physical fitness means an easy superiority - and not a gun. Slavery and serfdom took other forms, so are associated stereotypes. Talking while seemingly alone is, arguably, not a solid sign of a mental illness now. First paleness became no longer a wanted trait, then we learnt that sun tan can be bad too. Putting fire to a field or a property isn't a good idea like it was before. Natural resources are free, limitless and harvested with no consequencies. Finding a stash of gold isn't that tempting too. Mass production, services kind off changed the amount of skills one needs in an average household and added complexity to it. Knowledge of how to get a clean water noticeably changed our ways. And perception of sex and family in different cultures drastically changed over time due to religion, law and science.

Whatever you do, don't ask for bloodletting if you get sick