Tesla Inc's shares dropped the most in two years on Friday, rocked by the departure of its accounting chief after just a month and mounting investor concerns about chief executive Elon Musk's behavior after he smoked marijuana on a live webcast.

Investors in the electric carmaker are on edge after a tumultuous August, during which Musk proposed and then abruptly pulled the plug on a go-private deal.

Tesla said chief accounting officer Dave Morton resigned because of discomfort with the public attention and pace of work. Morton, whose departure comes after the US Securities and Exchange Commission opened an inquiry into Musk's aborted plan, joins a list of executives who left Tesla recently.

Bloomberg also reported on Friday that chief people officer Gaby Toledano would not return from a leave of absence just over a year after joining. 

Even before Musk’s surprise August 7 Twitter disclosure that he had funding "secured" for a go-private deal, Tesla had been under scrutiny from investors, analysts and short-sellers as it works to hit production targets and slow its cash burn.

Morton, who joined Tesla on August 6, was quoted in a company filing on Friday saying he believes "strongly" in Tesla and that he had no disagreements with the company's leadership or its financial reporting.

Tesla's $1.8 billion junk bond maturing in August 2025 plunged as much as 4 cents on the dollar to below 82 cents, a record low, in Friday trading, pushing the yield above 8.8 percent.

The bond had tumbled earlier this week as Musk renewed an attack on a British caver whom he had previously insulted on social media a day after Mercedes-Benz unveiled a challenge to the electric car maker.

Late on Thursday, Musk was filmed smoking marijuana and drinking whiskey in a 2-1/2-hour live web show with comedian Joe Rogan that swiftly spread across Instagram and other social media. Recreational marijuana is legal in California.

Tesla shares, down just 1 per cent after Musk's appearance on the Rogan podcast overnight, fell as much as 10 per cent after Morton's resignation and were still down 5.1 per cent at $266.75 in late morning trading.