BOSTON (Reuters) - Billionaire investor William Ackman walked away from Valeant Pharmaceuticals International Inc on Monday with a loss of more than $3 billion as he sold his entire stake in the struggling drug company after trying to rescue it for some 18 months.

The abrupt and unexpected move by the powerful activist investor sent Valeant shares tumbling almost 10 percent in after-hours trading. They have lost 95 percent of their value since mid-2015.

For Ackman, it marked a dramatic climbdown from his vocal support of the company, but should help soothe his own investors who had begun to show signs of concern about mounting losses in his portfolio.

“We elected to sell our investment and realize a large tax loss which will enable us to dedicate more time to our other portfolio companies and new investment opportunities,” Ackman said in a statement.

Ackman’s Pershing Square Capital Management became one of Valeant’s biggest investors in 2015 when it sunk some $3.2 billion into the company. At its peak the Valeant stake was worth roughly $4 billion.

Pershing Square said on Monday the Valeant position, at its current market value, represented 1.5 percent to 3 percent of its various funds.

Already one of the hedge fund industry’s most vocal investors, Ackman turned himself into Valeant’s biggest cheerleader and fixer, even as the stock price plunged amid U.S. regulators’ probe of Valeant’s pricing policies and problems at its specialty pharmacy unit, Philidor.

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After securing a board seat, Ackman replaced the chief executive, overhauled the board of directors and made some asset sales.

But the biggest move - trying to sell Salix, the company’s gastro-intestinal division, to Japanese company Takeda - eluded Ackman after advanced negotiations failed to lead to a sale.

He sold Pershing’s 18.1 million shares of Valeant on Monday, plus about 8.8 million under his own name, together representing almost 8 percent of Valeant overall, according to Reuters data.

BIG DROP

Ackman’s fund bought into Valeant when the stock was trading near $190 a share and he watched it surge to $260 a share during the summer of 2015. But regulatory scrutiny and other concerns caused the stock price to sharply tumble after August 2015.

The stock has fallen 16 percent since January even as many other stocks have been buoyed by hopes of stronger economic growth and increased merger activity.

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The shares closed at $12.11 on the New York Stock Exchange on Monday, and dipped to $10.93 in after-hours trading.

That long decline has tarnished Ackman’s reputation as an investor and wreaked havoc on his portfolio.

After gaining 37 percent in 2014, his Pershing Square International Fund lost 16.6 percent in 2015 and 10.2 percent in 2016, largely because of the Valeant losses.

Monday’s move mirrored a similar exit in the summer of 2013 when Ackman sold his entire stake in retailer J.C. Penney and stepped off the board after having failed to fix the company.

“Ackman never, never gives up, at least not until a year or two after everyone else has given up,” said Erik Gordon, a professor of law and business at the University of Michigan.

Pershing Square was Valeant’s second-largest owner after hedge fund Paulson & Co, a regulatory filing shows. Hedge fund ValueAct Holdings is the third-biggest owner.

For Ackman’s investors - pension funds across the country and wealthy private investors - the losses were beginning to wear and speculation had been mounting that they might not endure another year of declines. Ackman’s gains at the start of the year have already turned into losses.

It takes two years for investors to exit Pershing Square Capital Management, but Ackman has protected himself by building permanent capital of roughly $6 billion, which should ensure that his roughly $12 billion hedge fund can endure some investor departures.