It is well known that Abraham Lincoln took a medicine called "blue mass" or "blue pill," commonly prescribed in the 19th century. What is now hardly known is that the main ingredient of blue mass was finely dispersed elemental mercury. As his friends understood, mercury was often prescribed for melancholy or "hypochondriasis," a condition Lincoln famously endured. Mercury in the form of the blue pill is a potential neurotoxin, which we have demonstrated by recreating and testing the recipe. We present the testimony of many of Lincoln's contemporaries to suggest that Lincoln suffered the neurobehavioural consequences of mercury intoxication but, perhaps crucial to history, before the main years of his presidency; he was astute enough to recognize the effects and stop the medication soon after his inauguration.