Canadian marijuana producer Cronos Group's stock never should have been valued as high as it is now, according to short-seller Andrew Left.

"Why would I not be short the stock? It's a very competitive industry. I think they are a subpar player. I think they have a lot of issues," Left said on CNBC on Thursday evening after putting out a negative note earlier in the day.

"They've over-promised, and I think they're way out of whack with the rest of the industry," Left argued on "Fast Money."

Shares of Cronos tumbled 28 percent on Thursday following a report published by Left's investment newsletter Citron Research. Early Friday on Wall Street, the stock was recovering about 5 percent of that decline. Cronos Group owns medical marijuana growing and distribution operations in Australia, Canada, Germany and Israel — all countries where medical marijuana is legal. The company made history in February by becoming the first "plant-touching" cannabis company, or company that deals directly with the cannabis plant, to list on a major U.S. exchange.

Citron believes Cronos is only worth $3.50 per share, about one-third of its closing price Thursday of just over $9.

The company "appears to have been deceiving the investing public by purposely not disclosing the size of its distribution agreements with provinces," Citron alleges.

Cronos told CNBC it does not respond to short-seller comments.

"We can assure the public our securities offerings have been underwritten by reputable banks and our respected advisers have done all the necessary due diligence under both U.S. and Canadian securities law," Cronos said in a statement.

"Citron would like to inform investors of caution on the ongoing and real green rush," Left's report said. "Although the hype is big ... there are over 100 licensed producers [in Canada] and there will ultimately be more losers than winners."

Left said he'd still value Cronos at $3.50 per share, regardless of whether there are "problems" or not.

"If I want to give them the same multiple as all their peers, which I don't think have the same issues they have, the stock is $3.50," he said. "That's being generous to them."

But Left also said to keep things in perspective.

"Let's put it this way, the stock is up ... like 400 percent this year. It's not like anything is crazy. It should have never been where it was [Wednesday], and it should never be where it is right now," he said.

Short selling is a practice in which traders can bet against a company by selling shares they don't own and buying them back at a lower price.