The Lancet suggests it's arsenic poisoning:

Far from making him better, the medication used to treat the madness of King George III may actually have made him worse, according to research published on Friday.

One of the longest serving British monarchs who ruled for nearly 60 years, George had five very public bouts of madness culminating in his death -- blind, deaf and insane in January 1820.

The generally accepted theory has been that his fits of insanity -- the best documented lasting from October 1788 to February 1789 and triggering a constitutional crisis -- were due to a genetic disorder that caused variegate porphyria.

But there was no explanation of why the disease that causes symptoms such as lameness, hoarseness, acute abdominal pain, insomnia and temporary mental disturbances hit so late in his life or why the bouts were so deep and lasted so long.

Now a team of scientists from Britain and Australia have found high concentrations of arsenic in samples of the king's hair and suggested it came from the antimony-based medicine administered -- sometimes by force -- to cure him.

"If George III did indeed inherit the inborn error of metabolism that causes porphyria, he would be sensitised to the effects of arsenic and other heavy metals," they wrote in Friday's issue of the Lancet medical journal.

"Indeed, amounts of toxic metal insufficient to induce frank poisoning would, in all probability exacerbate porphyric attacks in a susceptible individual," they added. "The toxic metal most abundant in our findings was arsenic."

(Lovell, "Medication triggered madness of King George", Reuters, Jul.22)