(Reuters) - Billionaire investor Carl Icahn has acquired a small stake in Allergan Plc AGN.N at a time when the drugmaker is under pressure from other activist shareholders, people familiar with the matter said on Wednesday.

FILE PHOTO: Billionaire activist-investor Carl Icahn gives an interview on FOX Business Network's Neil Cavuto show in New York, U.S. on February 11, 2014. REUTERS/Brendan McDermid/File Photo

Icahn’s intentions are not clear, though he has been a longtime supporter of Allergan Chief Executive Brent Saunders, pushing for him in 2013 to become CEO of Forest Laboratories, which through mergers and acquisitions became Allergan.

Icahn, who had invested in Allergan in the past, believes the company’s stock is undervalued, the sources said, asking not to be identified because the matter is confidential.

Icahn did not respond to a request for comment.

“We welcome all investments in the company,” Allergan spokeswoman Amy Rose said.

Bloomberg News first reported on Icahn’s new position in Allergan.

Earlier this week, investment firms Appaloosa LP and Senator Investment Group LP wrote a letter to Allergan’s board of directors urging it not to pursue transformative acquisitions but instead shake up its board, including separating the roles of CEO and chairman, which are both held by Saunders.

The letter came shortly after Allergan said it had concluded a strategic review it launched earlier this year.

As a result of the review, Allergan is exploring divesting its women’s health and infectious disease businesses, opting to focus on areas where it has strong leadership positions.

It also said it would pursue a more disciplined capital allocation strategy.

Icahn last showed his support for Saunders in 2016, when he unveiled a stake in Allergan after its planned $150 billion sale to Pfizer Inc PFE.N was shot down by changes the U.S. government implemented to the rules on corporate tax inversions.

In 2016, Allergan sold its generic drug business to Teva Pharmaceuticals Inc for about $40 billion. In April, Allergan said it was considering an offer for rare disease drug maker Shire Plc SHP.L, only to reverse course after Allergan shares plummeted on the news.

It has spent much of the money on stock buybacks, debt reduction and a series of small-to-midsized acquisitions, including medical technology company ZELTIQ Aesthetics Inc and drugmaker Tobira Therapeutics.