If Lincoln hadn't stopped the medication, his steady hand at the helm through the Civil War might have been considerably less steady

Dr Norbert Hirschhorn

One such episode occurred during a debate which took place in 1858.

It is recorded that Lincoln reached over and picked a man up by the coat collar at the back of the neck and shook him "until his teeth chattered".

He became so angry "his voice thrilled and his whole frame shook."

Erratic behaviour

A new study suggests that Lincoln's erratic behaviour may have been linked to the pills he took to treat his persistent "melancholia".

The pills, known as blue mass, contained the metal mercury in potentially toxic doses that exceeded modern day safety limits by nearly 9,000 times.

The researchers believe that this toxic effect may have had a profound impact on Lincoln's mood.

Lead researcher Dr Norbert Hirschhorn said: "We wondered how a man could be described as having the patience of a saint in his fifties when only a few years earlier he was subject to outbursts of rage and bizarre behaviour."

His colleague Dr Robert Feldman, from Boston University, added: "Mercury poisoning certainly could explain Lincoln's known neurological symptoms: insomnia, tremor and the rage attacks.

"But what is even more important, because the behavioural effects of mercury poisoning may be reversible, it also explains the composure for which he was famous during his tenure as president."

Gave up medication

Lincoln stopped taking blue mass in 1861, a few months after his inauguration as president, having noted that the pills made him "cross".

The researchers set out to determine how toxic the mercury in the blue mass pills was likely to be.

They used a nineteenth century recipe to recreate blue mass.

The ingredients included, besides mercury, liquorice root, rose-water, honey and sugar and dead rose petals.

The researchers found that the solid element of mercury absorbed from two pills would have been 750micrograms.

The US Environmental Protection Agency indicates that only up to 21micrograms of any form of mercury per day may safely be ingested.

Someone who ate the common dose of two to three little pills per day would have seriously risked poisoning.

Dr Hirschhorn said: "The wartime Lincoln is remembered for his self-control in the face of provocation, his composure in the face of adversity.

"If Lincoln hadn't recognised that the little blue pill he took made him 'cross,' and stopped the medication, his steady hand at the helm through the Civil War might have been considerably less steady."

The research is published in the journal Perspectives in Biology and Medicine.