Another, much more common example: Tylenol. To date, it is still not fully understood how paracetamol/acetaminophen works. Due to the fact that it was sold lon

Another, much more common example: Tylenol. To date, it is still not fully understood how paracetamol/acetaminophen works. Due to the fact that it was sold long before understanding the mechanism of action was required by the FDA, and there are little adverse effects in the history of the drug, it's permitted to be sold today.