One of the most interesting aspects of the medical world is how the evolution of medical knowledge rapidly transforms the types and availability of treatments for various conditions in a home pharmacy, and drugs that did not work initially can be repurposed for other treatments.
The starkest example of this was the drug thalidomide, which was the cause of the greatest man-made medical disaster in history to a potent treatment for leprosy, albeit not the first line one.
This can also work in reverse, as many medicines that were commonly used, prescribed and even sold over the counter have since become highly illegal, with exceedingly slight exceptions.
Cocaine
The active ingredient of the coca leaf and part of the reason Coca-Cola got its name, cocaine was seen as a highly potent painkiller that could be used as an anaesthetic, a stimulant, and (rather ironically), a treatment for morphine addiction, an opiate in the same family as heroin.
It could be bought as a cure-all for basically any ailment, appearing as a powder, a tablet, a cigarette additive and a tonic, but its now-infamous side effects often caused the problems it was advertised as curing.
Ultimately it was banned in 1920 and is currently a highly illegal Class A drug.
Heroin
In the late 19th century, the biggest social vice of Victorian London was morphine, a powerful opiate (a byproduct of opium, derived from the poppy) that was itself marketed as a treatment for opium and alcohol addiction whilst being significantly more addictive than either of them.
Heroin, a synthesised form of morphine that was five times as effective and allegedly less addictive, was initially sold in 1895, and within three decades had been made illegal due to its highly addictive qualities, although it is available on prescription in exceptionally rare and controlled circumstances.
Methamphetamine
Originally synthesised in 1893 by Nagai Nagayoshi, it would take several decades before crystal meth was used as a medicine, primarily as a stimulant used by German armed forces to enable extended periods of wakefulness.
However, within just a few years the extreme withdrawal symptoms were discovered, and yet at the same time, it was marketed as a diet pill in the 1950s and 1960s until being banned in the 1970s due to its dangerous side effects.