Valeant Pharmaceuticals International Inc. tumbled to a two-year low, closing out its worst month ever, after billionaire shareholder William Ackman failed to persuade investors the stock was undervalued and short-seller Citron Research hinted at fresh allegations of wrongdoing.

With new questions surfacing about the drug company’s business practices, Ackman, the chief executive of Pershing Square Capital Management, offered a full-throated, and at times rambling, defense of Valeant, saying its stock would bounce back and eventually soar to new heights. His prediction: $448 by 2019. The stock hit $103.84 before a tweet by Citron Research at 11:25 a.m. Friday sent it tumbling to $93.77 at the close, a 16 percent drop for the day.

The overall picture that Ackman, and his coterie of legal and financial advisers, sought to paint was one of a misunderstood and occasionally bumbling, but essentially decent business. He dedicated little time to exploring the question of Valeant’s relationship with Philidor, the pharmacy at the center of the recent scandal rocking Valeant’s shares, and instead lashed out at what he characterized as an inept public and investor relations effort by Valeant. Particular criticism was leveled at a Valeant PR adviser, who Ackman described as having done a "horrible job."

"Ackman cannot know what Valeant is really doing and this call proved that," said Bill Kavaler, a New York-based managing director at brokerage firm Olivetree Financial. "There is nothing he said that isn’t already known."

He also blamed rival drugmakers for stirring the pot when it came to Valeant’s reputation, suggesting envy at Valeant’s success had contributed to the company being cast in a negative light during recent weeks. "There are a lot of people in the pharma sector who don’t like" Valeant CEO Mike Pearson, Ackman said. That’s "because it is embarrassing for him to have twice the operating margins they do. So there is a lot of whispering."

Valeant, a Wall Street darling for years, traded over $260 back in August and was hovering at $146 last week, when Citron Research, run by the short seller Andrew Left, questioned the company’s accounting practices. Left took another poke at Valeant and Ackman while he spoke today, saying that the drugmaker was more likely than Herbalife Ltd. -- a stock Ackman has been betting against for years -- to fall to a stock price of zero. The comments, along with Left’s pledge to reveal further information on Monday, deepened the stock’s declines.

Valeant shares closed the week down 19 percent and ended October down 47 percent -- the worst month on record -- closing at the lowest price since July 31, 2013.

In an interview with Bloomberg News today after the call, Left mocked Ackman’s assertion that he was unaware of Valeant’s full relationship with the mail-order pharmacy that has unnerved investors, as well as Ackman’s contention that Valeant would overcome its current problems.

“How crazy is that?” Left, 45, said of Ackman’s comments. “That tells you he doesn’t know the business very well.”

Ackman in the call sought to discredit Left’s claims that Valeant could be a modern-day Enron Corp. He repeatedly drew comparisons between the drug maker and Berkshire Hathaway, the acquisitive holding company of storied investor Warren Buffett. His efforts to link the two companies became so frequent that one person who dialed into the call said Ackman was trying to wrap "himself up in Buffett’s clothes." Valeant, while cutting ties today with Philidor, has denied Left’s claims that its accounting is fraudulent.