The premium is worth as much as $600 million, compared with what Pfizer could get for its own shares at today’s closing price of $29.10. Pfizer announced today that investors will get 0.9898 shares of Zoetis for each Pfizer share they swap and have until June 21 to decide whether to participate.

The deal will complete the current phase of Chief Executive Officer Ian Read’s plan to shed non-drug units and focus Pfizer on developing and selling new, brand-name therapies. The New York-based company is next looking at whether it will split itself and get rid of its generics drug business as well, which could happen in the next three years.

The exchange announcement almost completes the spinoff of the animal health company. Because of the way the deal is structured, investors who offered shares for exchange will get two more days after Wednesday’s original deadline to decide whether they want to turn them in, Pfizer said in a statement.

The drugmaker has held 80 percent of Zoetis since an initial public offering in January. If investors trade their shares for all of Pfizer’s stake, it will cut $12 billion in outstanding shares.

Shares Offered

Pfizer is expected to announce next week how many shares are being exchanged. Before the two-day extension, investors had offered 868 million shares of Pfizer, more than double the amount the company has in Zoetis shares to give out. Investors will get a pro-rated amount of Zoetis stock, the company has said.

The exchange ratio was based on an average price of Pfizer and Zoetis shares, with 0.9898 being the upper limit for the swap.

The transaction is designed to give Pfizer shareholders an incentive to swap part of their holdings and let the drugmaker close out its stake in Zoetis. Under the terms, Pfizer shareholders are expected to get about $105 worth of Zoetis for every $100 of Pfizer stock accepted in the exchange.

Zoetis is based in Florham Park, New Jersey, and makes and sells drugs and treatments for animals. Livestock products make up 65 percent of its sales, while medicines for pets account for the rest. Much of the company’s growth depends on rising wealth levels around the world, as people eat more meat and care for companion animals. Zoetis shares fell less than 1 percent to $31 at the close.

When the transaction is done, it will give Pfizer what is essentially a multibillion-dollar share buyback, and will help boost earnings per share by cutting Pfizer’s share count. Pfizer sold a 20 percent stake of Zoetis in January, in an IPO worth $2.24 billion.

JPMorgan Chase & Co., Bank of America Corp., Goldman Sachs Group Inc. and Morgan Stanley managed the deal. Skadden, Arps, Slate, Meagher & Flom LLP acted as legal adviser.