Bill Ackman said Friday that Valeant Pharmaceuticals made a mistake by underinvesting in communications.
"Investor relations and government relations are not areas the company has meaningfully invested in," Ackman said. "Investors are willing to accept complexity as long as there is transparency."
He also said he is a major shareholder in the company.
Shares of Valeant traded sharply lower on Friday.
Ackman, founder and CEO of hedge fund Pershing Square Capital Management, made his remarks in a conference call with investors held Friday to discuss his investment in the drug company.
On Monday, Valeant laid out a detailed defense of its relationship with the little-known specialty pharmacy Philidor Rx Services, but its arguments failed to calm all investor concerns. Valeant announced earlier Friday that it had cut its ties with Philidor.
Valeant shares have plunged in the wake of a report from influential short-seller Citron Research that suggested the Canadian drugmaker may have fraudulently inflated revenues by using its ties with a specialty pharmacy.
Ackman said Citron's claim on the drugmaker is "verifiably false." He also swept up 2.1 million shares after Citron's report was released.
Still, Alan Hoffman, executive vice president at Herbalife, a company in which Ackman has a short position on, said in a Friday statement: "I hope Bill Ackman has done more research on Valeant than he did on Herbalife, Target, Borders and J.C. Penny."
—Reuters contributed to this report.