Elon Musk has said the past year of his professional life has been “excruciating” and that stress over his business had caused his health to deteriorate. To make matters worse, the confession wiped billions off the value of Tesla, the electric car company he founded.

In an emotional interview with the New York Times, the founder of electric carmaker Tesla also revealed that the pressures of work had caused him to spend his birthday stuck in the Tesla factory and almost miss his brother’s wedding.

“This past year has been the most difficult and painful year of my career,” he said in the interview.

Tesla’s shares fell close to 9% on Friday, wiping over $5bn off the company’s value.

Musk, whose company posted a record $709.6m net loss in the first quarter of this year while struggling to manufacture large numbers of its Model 3 mass-market electric car, said he had been working up to 120 hours a week and his health had been “not great”. He said his friends had been worried about him.

Musk said there were times he didn’t leave the factory for three or four days at a time and days when he didn’t go outside.

He said he spent the entire 24 hours of his 47th birthday in June at work and flew directly to his brother’s wedding, where he was due to be best man, from work, arriving two hours before the ceremony and leaving immediately afterwards.

Am considering taking Tesla private at $420. Funding secured. — Elon Musk (@elonmusk) August 7, 2018

Musk sparked a furore in the markets, the media and among investors earlier this month when he tweeted his intentions to take his publicly traded company private, with the assurance: “Funding secured.”

Lawyers have warned that his tweet may have broken rule 14e-8 of the Securities Exchange Act of 1934, which prohibits publicly traded companies from announcing plans to buy or sell securities if executives don’t intend to complete, don’t have the means to complete, or are trying to manipulate the stock price.

Tesla’s stock rose by 11% after the tweet. But last week, Musk was sued by investors, who claimed his tweets were misleading and were designed to “completely decimate” short-sellers, people who bet that Tesla’s shares will lose value. It has also been reported that the US’s top financial watchdog had launched an investigation into Musk in light of his tweets.

In the interview with the New York Times, Musk blamed short-sellers, from whom he said he anticipated “a few months of extreme torture”.

“I thought the worst of it was over,” he told the New York Times. “But from a personal standpoint, the worst is yet to come.”

He denied “being on weed” at the time of the tweet and said he saw it as an attempt at transparency, but admitted in the interview on Thursday that no one had seen or reviewed it before he posted it. Observers puzzled over the price he cited.

Musk explained that with an estimated premium over recent values, the share price should be around $419. He decided to round it up to $420, he said – a number that is slang for cannabis.

“It seemed like better karma at $420 than at $419,” he said in the interview. “But I was not on weed, to be clear. Weed is not helpful for productivity. There’s a reason for the word ‘stoned’. You just sit there like a stone on weed.”

Musk, who is a controversial figure, has drawn further criticism this year for his tweets. Last month he launched an extraordinary attack on one of the British divers involved in the rescue of 12 boys and their football coach who were trapped in a cave in Thailand, baselessly calling the diver a “pedo” on Twitter.

In June, Musk caused outrage when he responded to a tweet from a woman who claimed Musk had used her father’s unicorn artwork on Tesla merchandise without compensation or attribution, by telling her: “He can sue for money if he wants, but that’s kinda lame.” Musk later settled the dispute with the artist.

Musk has also lashed out at both the media and analysts from large Wall Street banks in recent months, calling questions from analysts “boring, bonehead questions” and suggesting he would start a website to rank the credibility of individual journalists, after Tesla received negative press coverage.