Mercury--A Cure Until it Killed You
One of the fun parts about writing time travel is the fact those from the future know things those from the past have yet to figure out. One of my characters in Time-Crossed Christmas tries to stop a Civil War doctor from giving a young syphilis patient a healthy dose of mercury or calomel (mercurous chloride.) After all, mercury was considered a cure--until it killed you.
Of course, we know it's a toxic substance today, but for many centuries, and certainly in 1863, it was a cure-all treatment for countless ailments from depression to yellow fever. It was often given until a patient began salivating (or their hair and teeth fell out) which proved to them it was ridding the body of impurities and restoring the balance of humours. One doctor recommended two pints of saliva as the appropriate response to a winning dose!
Since any quack could formulate a compound and calomel was a known ingredient, it showed up in just about every tincture, pill, or balm sold. Everyone from mad King George III to Abraham Lincoln were treated with it. In fact, the daily dose of mercury that Lincoln took in the form of blue mass would have exceeded the current EPA safety standards by nearly 9,000 times!
It wasn't until the mid 1950s and the development of penicillin that mercury was finally recognized as toxic enough for the substance to be banned in most ingested forms, and eventually even as a substance in fever thermometers.