The buy will be funded by a placement and share issue which will lift Mayne's sharemarket worth by more than 40 per cent to close to $2 billion. The Australian Financial Review's Street Talk column revealed the raising details on Monday night.

Mayne Pharma has built a small presence in the US drugs market over the past few years, although the deal with Teva catapults it into the ranks of the top 25 generic drugs groups in that market. Generic drugs are cheaper versions of longstanding drugs which have lost patent protection, which opens them up to competition from more competitively priced rivals.

"The acquisition transforms Mayne Pharma's generic products division into a top 25 player in the US retail generics market, diversifying Mayne Pharma's earnings across a broad range of products, therapeutic areas and technologies," Mayne's chief executive, Scott Richards said.

The acquired portfolio is expected to add $US237 million to sales in fiscal 2017, it said, with gross margins greater than 50 per cent.

The purchase price represents a multiple of less than six times the projected gross profit, as measured by earnings before interest, tax, depreciation and amortisation, of the acquired portfolio.

The deal follows the planned purchase by Teva of Allergan, which has forced the divestiture of a range of products, which has opened the door for Mayne to lift its presence in the US market.

Mayne Pharma said it will raise $287 million via a placement and turn to shareholders for a $601 million rights issue on a ratio of 1-for-1.725 shares held. Shares in the placement are to be issued at $1.50 each, with the shares issued in the rights issue priced at $1.28, a 9.2 per cent discount to Monday's close.

The deal is the last of several large divestitures of US assets required by regulators for the approval of Teva's planned $US40 billion acquisition of Allergan's generic drugs portfolio. ​Earlier, Teva has agreed to sell assets to Dr. Reddy's Laboratories, Impax Laboratories, Sagent Pharmaceuticals and Cadila Healthcare, along with several privately owned companies. The US asset divestitures are worth around $US2 billion in total.

Mayne Pharma said it is acquiring 37 products already approved for sale with a further five products under application for sale. It intends to transfer production of 11 of the products to its own units and continue to outsource the balance.

The focus of the acquired drug portfolio is on oral contraceptives, central nervous system and cardiovascular products, it said, with 59 per cent of the products approved for sale having two or less generic competitors.