Betting against cannabis stocks is becoming an increasingly expensive occupation.

Short sellers have lost $490 million year-to-date betting against the sector and $626 million since the start of August, according to S3 Partners, as pot stocks rallied off the legalization of cannabis in some countries and investment interest from some of the world's biggest beverage companies. (See also: Coca-Cola May Foray Into Cannabis-Infused Drinks.)

Research from the financial analytics firm showed that an increasing number of investors responded to excitement surrounding the sector’s prospects by betting against 33 stocks and exchange-traded funds (ETFs) tracking the industry. Short interest in the cannabis space reportedly increased by $458 million, or 44%, to $1.5 billion since the end of the second quarter.

S3 Partners added that a large chunk of this rise was centered on two of the sector’s biggest gainers: Tilray Inc. (TLRY) and Canopy Growth Corp. (CGC), whose share prices have been on a tear since Constellation Brands Inc. (STZ.B) announced a massive $4 billion deal last month to increase its ownership stake from less than 10% to nearly 38% in the company.

Aside from being hit with continued share price rises, short sellers are also having to fork out an increasing amount on fees to bet against the marijuana industry. Investors bet against companies by borrowing their shares and then selling them, in anticipation that they can later buy them back at a lower price. Those lending the shares charge a fee, which tends to go up as the stock becomes harder to get hold of.

S3 Partners reported that short sellers are paying over $2.4 million per day in financing costs as the average fees to borrow stocks in the sector hit 21.8%. The report also added that shorting the two main cannabis ETFs isn’t much cheaper, costing investors on average a fee of 20.8%. (See also: Marijuana ETF Attracts $22 Million in August)

Higher average fees were blamed in part on the difficulty to locate stock borrows for some of the sector’s most shorted shares. Shorting Tilray’s stock now reportedly commands a fee of anywhere between 450% to 600%. The Green Organic Dutchman Holdings Ltd. (TGOD) and Cronos Group Inc. (CRON) were also identified as particularly expensive stocks to short, with fees hovering close to 50%.