Folk remedies have fallen out of favor largely because science has proven that many of them are ineffective or dangerous. For example, mercury was a component of many medicines until the early 20th century, at which point scientists discovered that it is toxic to the human kidneys, liver and brain.
Pharmaceutical companies test modern medicines extensively in order to prove how they work and look for potential toxicities and side effects. By contrast, many medicines of old worked in ways that were poorly understood, if they worked at all. Some were based completely on superstition, such as an ancient Babylonian cure that called for kissing a skull seven times before bed to quell the spirits of dead family members who were causing a person's teeth to grind at night.
One of the most extreme examples of an ancient remedy that was both gruesome and dangerous is trepanation, which is the practice of boring holes in the skull to cure illnesses. Practiced as recently as 7,000 years ago, trepanation may have been a primitive form of head surgery, meant to alleviate mental afflictions or physical problems such as blood clots and bone fragments in the skull. Surprisingly, evidence exists that some trepanned patients survived.