Tesla Inc. shares TSLA, +3.81% sank more than 8% Friday as investors digested the news of a widening Securities and Exchange Commission probe of Chief Executive Elon Musk’s handling of a tweet disclosing his wish to take the company private.

Musk told The New York Times that he had no regrets about saying “funding secured” in a tweet that said he was considering a going-private deal at $420 a share. In an interview with the paper published late Thursday, Musk said he has had a very difficult year spent in a constant state of exhaustion leading up to the tweet, which is understood to have angered some members of the board.

Related: Wall Street has spoken: Tesla funding is not ‘secured’

The Times said Tesla’s board was concerned about Musk’s workload and health, especially his use of the sleep aid Ambien, and that efforts are under way to find a second-in-command who could take some of the strain off of him.

Tesla stock ended at its lowest since Aug. 1, down 8.9% at $305.50. Weekly losses topped 14%, its largest since the week of Feb. 5, 2016, when it slid 15%. Tesla shares also relinquished this year’s gains; they are down 1.9% in 2018, vs. a gain of 6.6% for the S&P SPX, +0.67% .

Musk told the Times he has no plans to stop tweeting. The SEC is investigating the matter, and Musk could face charges if he was found to have intentionally misled investors in his tweet.

He also addressed why he said the stock price for going private would be $420 a share — a reference that some at first thought was a joke, referring to marijuana.

Musk said he wanted to price the stock at a 20% premium from its price at the time — which would have been $419 a share. So he rounded up. “It seemed like better karma at $420 than at $419,” he told the Times. “But I was not on weed, to be clear.”

Musk said he has recently been working 120-hour weeks, and sometimes would not leave the factory for days on end — including his 47th birthday in June. But while “production hell” appears to have peaked, Musk said his struggles are not over.

“The worst is over from a Tesla operational standpoint,” he said. “But from a personal pain standpoint, the worst is yet to come.”

Claudia Assis in San Francisco contributed to this report

More on Elon Musk and Tesla:

Elon Musk left plenty of questions about Tesla going private, experts say

Tesla will lose money on a cheaper Model 3, analyst says

Elon Musk’s plan to take Tesla private is a pipe dream

Elon Musk is a ‘natural maniac’ — here’s what it means for Tesla investors, in one chart