He told CNBC's "Fast Money: Halftime Report" he thought seriously about selling his stake in the drugmaker, but decided he could "fix it." Ackman's holdings have endured a dismal year, hit in particular by the stake in Valeant.
His Pershing Square Capital Management portfolio had a return of -17.5 percent this year as of April 26.
Valeant shares had lost about 9 percent Monday but cut the losses in half as Ackman spoke. He touted changes in the company's structure and leadership, like its new CEO Joseph Papa.
Ackman said he aims to create a "meaningful" turnaround for the company. He added he believes Valeant has "made appropriate changes in personnel."
Ackman noted that Valeant is not considering selling any "core" assets, despite interest from buyers.
The drugmaker on Friday filed its long-delayed annual report, helping it prevent a default on its $30 billion in debt. It pointed out misstatements that would cut some of its previously reported sales.
During a Senate hearing last week on Valeant's drug pricing, Ackman said his first priority was to make sure the company did not go bankrupt.
Many politicians have recently looked to take more action on reining in increases in the price of drugs that aren't widely available. Ackman noted that price cuts at Valeant are "absolutely" on the table.
— Reuters contributed to this report.