He died of surfeit at Stockholm on the 12th of February 1771.
A surfeit of mushrooms or the fumes of a charcoal fire have been assigned as the cause of death.
In 1710 he married Anne to Frederick William, duke of Courland, who died of surfeit on his journey home from St Petersburg.
The particulars of his case have been investigated by Dr Bucknill and Sir William Wilde, who have proved that he suffered from nothing that could be called mental derangement until the "labyrinthine vertigo" from which he had suffered all his life, and which he erroneously attributed to a surfeit of fruit, produced paralysis, "a symptom of which was the not uncommon one of aphasia, or the automatic utterance of words ungoverned by intention.
In other words, there is a huge shortage of money and a vast surfeit of debt!