Some of the top 20 investors at Salix Pharmaceuticals Ltd (SLXP.O) are threatening to vote down a proposed deal to buy a unit of Cosmo Pharmaceuticals SpA (COPN.S), and are pressing Salix to consider selling itself instead, people familiar with the matter said on Tuesday.

Raleigh, North Carolina-based Salix, which makes drugs for gastrointestinal disorders, said in July it would merge with Cosmo's Irish subsidiary, a deal that would allow Salix to move its tax domicile abroad in a practice known as inversion.

However, some Salix shareholders are unhappy with the plan and would rather see a sale to a larger drugmaker such as Actavis Plc (ACT.N) or Allergan Inc (AGN.N), the people said.

Investors holding at least a quarter of Salix shares are considering voting the deal down, said the people, who declined to be named because the matter is private.

The investors think that an inversion with Cosmo would make it difficult to find a buyer for Salix. The deal is subject to majority approval by Salix shareholders at a meeting expected later this year. A date for a vote has not been set.

A spokesman for Salix could not be reached for comment.

Salix shares traded down 3 percent on the day the deal was announced, reflecting investor disappointment the company was likely not a near-term acquisition candidate.

UBS analyst Marc Goodman called the transaction "a disappointing inversion deal" expected to be dilutive to 2015 earnings and whose benefits were years away.

Salix shares have soared 38 percent in the last three months on rumors it is a potential target for Allergan as the Botox maker tries to fend off a bid from Valeant Pharmaceuticals International Inc (VRX.TO).

Under the terms of the proposed deal, Salix shareholders would own 80 percent of a jointly held Irish unit and Cosmo would hold the rest. Cosmo is best known for two drugs to treat ulcerative colitis, a chronic disease that causes inflammation and sores in the lining of the large intestine.

If Salix terminated the Cosmo deal, it would only have to pay a $25 million break-up fee, relatively low given Salix's $9.2 billion market cap.

Tax inversions have been increasingly popular in the healthcare industry and have spread to other industries, drawing criticism from the U.S. government and lawmakers.

Last week, Treasury Secretary Jack Lew called for action to stem the surge of inversions. Senator Chuck Schumer, the No. 3 Senate Democrat, has circulated a draft bill attacking the "earnings stripping" often associated with the practice.

Inversions have been pursued by medical technology group Medtronic Inc (MDT.N), which said it would buy Dublin-based Covidien Plc (COV.N) for $42.9 billion, and fast-food chain Burger King Worldwide Inc (BKW.N), which seeks to acquire Canadian coffee chain Tim Hortons Inc (THI.TO) for $11.5 billion.