According to Norman Ohler, an award-winning German author, the Fuhrer became addicted to a heroin-like substance called Eukodel which was prescribed following a nervous breakdown in 1944.
Ohler's book 'Blitzed: Drugs in Nazi Germany', which British historians have praised as a "remarkable" work of research, argues that the heroin-like opiate was largely to blame for Hitler's erratic and paranoid behaviour towards the end of his life, The Telegraph reported.
"I cancelled injections today, to give the previous puncture holes a chance to heal," one entry reads.
"Left inside elbow good, right still has red dots (but not pustules), where injections were given," it says.
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Ohler said Hitler began the drugs regime after narrowly surviving the 1944 assassination attempt known as 'Operation Valkyrie', in which the German resistance planted a bomb in a briefcase under his desk.
"I'm afraid that from 1944 onwards, Hitler did not spend a single day sober," Ohler was quoted as saying.
"Before then, he was a very public person...But the attempt on his life left him withdrawn, paranoid and anxious. He demanded that Dr Morell restore him to his former confident self, so from that point on he received thousands of injections -- most frequently Eukodol, which is like heroin but with a greater potential to make you euphoric," he said.
"And it had found him, in its darkest hour, in Adolf Hitler," it says.
As the drugs took their toll, Hitler's decision-making became more erratic.
Antony Beevor, a British war historian, said the book's findings explained Hitler's "completely irrational" military tactics during the Battle of the Bulge, which was the dictator's last-ditch attempt to defeat the allies.
"All of these elements show how he was really no longer in control of himself, but he was still in control of the German armies," he told the BBC's Today programme.