Concerns about the billionaire Elon Musk's erratic behaviour intensified after the Tesla chief executive smoked a marijuana joint a live web show with US comedian Joe Rogan.

A video of the 47 year-old tech tycoon puffing tentatively on the large, cone-shape joint inevitably became a viral social media storm. Rogan passed the joint to the magnate, who took a single drag - marijuana is legal in California – but revealed he "almost never" smoked. "I'm not a regular smoker of weed," Musk said. "I don't actually notice any effect ... I don't find that it is very good for productivity."

Shares in Musk's electric car company Tesla plunged nine percent after the smoking pot podcast went public.

Musk spent two and a half hours on the streamed podcast with Rogan late Thursday discussing everything from artificial intelligence and its impact on humankind to flame throwers and social media.

In an email to The Guardian in response to a question about whether Tesla drug tests employees, Musk wrote: "Our policy allows trace amounts of THC during work times, provided they are below the safety limit (much like a minimum alcohol level)."

Earlier this week the PaylPal co-founder raised eyebrows with a renewed and bizarre attack on the British diver who helped save children from a Thailand cave system. Musk said, in no uncertain terms, that he still believes cave rescuer Vernon Unsworth to be a "child rapist" in a series of emails sent to Buzzfeed News at the end of August.

The profanity-laced emails were intended by Musk to be off-the-record responses to Buzzfeed after the publication asked Musk about possible legal action by Unsworth, though Buzzfeed did not agree to the conditions. The English caver was widely credited by others involved in the rescue operation as a vital part of the mission. He reportedly provided knowledge of the cave system in which 12 youth soccer players and their coach were trapped.

"I suggest that you call people you know in Thailand, find out what's actually going on and stop defending child rapists," Musk told Buzzfeed. The Tesla CEO then went on to encourage Unsworth to sue him, claiming the caver moved to Thailand for a 12-year-old "child bride."

Musk's initial accusations against Unsworth in July caused shares of Tesla to drop. On Wednesday morning, shares of Tesla (TSLA) fell more than 3 percent.

Musk has a reputation for making provocative comments, but his online conduct has raised caused and eroded investor confidence in recent weeks. In August, he tweeted that Tesla would go private, even going as far as to say the company had secured funding. Tesla would later abandon plans to go private, while Musk faced a possible SEC investigation for market manipulation.

Last month, Musk has complained about 120 hour working weeks and stress triggered by the demands of achieving the Tesla cars' production and financial targets.