Tesla shares dropped more than 8.5 per cent in early trading on Friday after the car maker’s chief executive Elon Musk gave an erratic interview during which he said the past year at the company had been “excruciating”.

Mr Musk told The New York Times that his health had been impacted by the stress of the last 12 months. “It’s not been great, actually. I’ve had friends come by who are really concerned,” he said.

Mr Musk repeatedly broke down in tears during the interview, the NYT reported, alternating between laughing and crying throughout.

He also admitted to taking sleeping pills, and said: “It is often a choice of no sleep or Ambien.”

He used the interview as an attempt to explain recent behaviour that has raised questions about his leadership at Tesla.

Last week, he sent the tech firm’s stock soaring with a tweet that said he was “considering taking Tesla private at $420. Funding secured”.

It later emerged that the tweet had not been approved by anyone else at the company and funding was not completely secured, and US financial regulators are investigating whether the tweet broke trading laws.

Meanwhile, the choice of 420 was noted as the number has become synonymous with cannabis use in the US.

However, Mr Musk said he decided on $420 because “it seemed like better karma at $420 than at $419”.

“I was not on weed, to be clear. Weed is not helpful for productivity,” he added. There’s a reason for the word ‘stoned.’ You just sit there like a stone on weed.”

Tesla and Mr Musk have been sued by investors over the claims about privatising the company, who allege that the entrepreneur engineered a scheme to squeeze short-sellers with his tweets.

Mr Musk also raised controversy via Twitter last month when he lashed out at a British diver who helped rescue schoolboys from a flooded cave in Thailand.