Call volume skyrocketed Thursday on Herbalife Ltd. (NYSE:HLF - 36.35), as approximately 70,000 contracts crossed the tape during the course of the session -- representing roughly four times the stock's average daily volume. The center of attention was the January 2013 37.50-strike call, where 15,802 contracts were traded. Most of these front-month calls changed hands closer to the ask price, suggesting they were purchased, and open interest here rose overnight by 11,572 contracts. Based on this evidence, it looks as though new calls were bought to open on HLF yesterday.

Those January 37.50 calls traded at a volume-weighted average price (VWAP) of $1.59. Speculators who picked up bullish bets at this price will need HLF to rally above breakeven at $39.09 (strike price plus VWAP) by the time front-month options expire, which occurs after the close of trading on Friday, Jan. 18. So, HLF has just a couple of weeks to gain more than 7.5%, based on Thursday's close at $36.35.

HLF has had a volatile ride on the charts lately, first sparked by the news that prominent investor William Ackman was shorting the stock. Just this morning, the shares were hit with a steep price-target cut to $54 from $82 at Wedbush, even as the firm maintained its "outperform" rating. Nevertheless, HLF jumped about 1% out of the gate.