Reuters/ Eduardo Munoz
Shares of Herbalife slipped below $45 on Tuesday. The stock was last trading around $44.69, down over 40% since the beginning of the year.
It's believed that Ackman shorted Herbalife in the mid-$40s back in 2012. CNBC's Kate Kelly reported that his average cost was $48.
It's difficult to know the break-even point for certain because in October 2013 he repositioned about 40% of his $1 billion equity short position for put options in an effort to reduce risk.
For nearly 20 months, Ackman has been on a crusade against Herbalife - a multi-level marketer that sells nutritional shakes and weight-loss products.
In late December 2012, Ackman gave a 342-slide presentation declaring that he was short $1 billion worth of Herbalife. It's Ackman's contention that the company is a "pyramid scheme" that targets poor people. His investment in predicated on regulators, specifically the Federal Trade Commission, shutting the company down. (The FTC opened an investigation into the company back in March.)
Right after Ackman's initial presentation, Herbalife's stock plummeted. However, soon after that, shares of Herbalife skyrocketed and Ackman began to rack up hundreds of millions in paper losses.
What's more is a number of other fund managers, most notably his long-time rival Carl Icahn, piled on by going long the stock. Daniel Loeb, another Ackman rival, briefly had a long position in 2013, but exited for a nice profit. Icahn, who reconciled with Ackman at a conference this summer, has said that he hasn't sold a single share of his massive Herbalife position.
Ackman has said before that people should not pay attention to Herbalife's day-to-day stock price. Perhaps he's noticed today's price movement, though.
Here's a chart going back to December 18, 2012-the trading session before Ackman publicly confirmed his short position.
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