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Investors shorting stocks related to the cannabis industry have lost $490 million on paper this year, but bets that pot stocks will fall have only increased despite high costs as the sector has rallied.

Massive gains in the industry among names such as the recently public Tilray Inc., Canopy Growth Corp. — which received a $4 billion investment from Constellation Brands Inc., helping fuel the sector’s recent rise — and Cronos Group Inc. have likely encouraged short sellers to take positions in cannabis stocks.

Short interest has increased 44% since the end of the second quarter, climbing to $1.5 billion across 33 stocks, according to data from financial technology and analytics firm S3 Partners. Most of the increase in short activity was focused in Canopy and Tilray, S3 said in a note released late Monday.

The number of investors interested in shorting the stock and the amount of interest has made short positions very expensive. According to S3 data, the average borrow fee for the basket of pot stocks is 21.8%.

Read more: Marijuana Stocks Just Keep Soaring Higher. But Even the Bulls See a Bubble

“On the whole, 20% is ridiculous,” said S3 managing director of predictive analytics Ihor Dusaniwsky. “It’s totally out of the ordinary, the normal fee for a general collateral stock — IBM, General Electric for example — is 30 basis points.”

Tilray especially has outsize fees for short sellers — those wishing to borrow the stock to short it were paying 450% to 600% Monday, Dusaniwsky said. Tilray is a special case, he says, because the float is relatively small and there is no institutional holding able to loan the stock to short sellers. As a result, Tilray’s gains — such as the 29% it posted in Tuesday trading after announcing a clinical trial in the U.S. — are almost entirely the result of the buy side bidding up or bidding down the market, Dusaniwsky said.

“I’m wondering whether [Tilray] is a Tesla or a normal stock,” Dusaniwsky said, referring to the fact that investors shorting Tesla Inc. stock have been willing to absorb massive losses while waiting for the stock to plummet. Roughly 34% of Tilray shares are currently being shorted.

Fees for other cannabis stocks trading on U.S. exchanges are also high, but nowhere near Tilray’s levels. Shorting Cronos stock carries a 40% borrow fee, and Canopy was 6%, according to S3 data.

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“Short sellers are paying $2.5 million every day at the moment in stock-borrow fees,” Dusaniwsky said. “If you’re going to short these stocks, your thesis has to be correct and it has to be right, fast.”

Cronos stock was up 16% Tuesday, and Canopy gained 6.3% as the S&P 500 index climbed 0.7%. Cannabis officially becomes legal in Canada on Oct. 17. It is illegal under U.S. federal law, though nine states have legalized the drug for adult recreational consumption and several more allow for medical use.

This article originally appeared on MarketWatch.