The European Commission has cleared U.S. pharma company AbbVie's proposed $63 billion acquisition of Ireland-based drug maker Allergan, provided the company sells one drug treatment currently in development.

Allergan will have to sell a product under development to treat inflammatory bowel diseases in order to secure final approval for the deal.

Margrethe Vestager, the EU's antitrust chief, said in a statement that inflammatory bowel diseases are lifelong conditions with devastating effects on the life of millions of people.

"Our decision makes sure that the merger between AbbVie and Allergan will not disrupt the development of a promising innovative treatment for these diseases," she said. "This will increase the choice of treatments and offer better prices for patients."

The Commission's investigation into the merger, which was notified in November, focused on treatments for inflammatory bowel diseases such as Crohn's disease where the two companies' activities overlap.

Allergan is developing a product that treats bowel diseases and which would compete with products in AbbVie's portfolio.

The Commission feared that the merger would have reduced innovation for drugs for inflammatory bowel diseases, as it was unlikely that Allergan would have continued developing its drug once the merger was approved.

"The transaction would have thus prevented a promising drug from reaching the market, leading to potentially less choice and higher prices for patients and health systems," the Commission said.

As a remedy, AbbVie has agreed to sell Allergan's drug, including the development, manufacturing and marketing rights at worldwide level, to a purchaser that will continue its development.