With $2 trillion in buyouts since January, 2015 has become the year of the megadeal, fueled by free money in a zero-interest rate environment.

But with the Fed appearing increasingly keen on hiking rates, that party may be coming to an end.

That is, unless you're in health care.

Almost one out of every four dollars in takeovers this year involved a company in health care, and the size of those deals is immense.

Compared with five years ago the total value of health care mergers and acquisitions in the United States has more than tripled, according to the data firm Dealogic.

Even in the face of rising interest rates, which would make deal-making a more expensive proposition, business insiders see few reasons why momentum in the health care sector will cool any time soon.

There is too much to gain.

Drugmakers still need to buy drugs and companies that promise to give them a sizeable competitive advantage and sell the ones that don't as they pare their focus. Health insurers, doctors and hospitals are pushing to get bigger and bring more muscle to their negotiations with each other over the ever-rising cost of care. The Blue-Cross Blue Shield insurer Anthem Inc. is spending $48 billion to buy rival insurer Cigna, a deal that would create a combined company pulling in well over $100 billion in revenue.

These deals also allow companies to combine resources and cut expenses at a time when every element of the sector is feeling pressure to control health care costs.

"Health care firms, in general, I believe will need to get bigger," Morningstar analyst Vishnu Lekraj said.

New York's Pfizer Inc. and Botox maker Allergan are discussing a deal that could easily top $100 billion and create the world's largest drugmaker based on revenue.

"If you're an investment banker, the (pharmaceutical) industry is still a good space," said Erik Gordon, a professor at the University of Michigan's Ross School of Business.

And then there's the long game.

Aging baby boomers are using more care, and a growing number of people are gaining insurance from the federal health care overhaul, and they're starting to return to the doctor's office.

On Friday, shareholders of the health care company Perrigo Co. rejected a hostile takeover attempt by generic drugmaker Mylan, an unsolicited, $26 billion offer that Perrigo called "grossly inadequate." Mylan NV quickly said it was ready to move on. Executive Chairman Robert J. Coury said in a printed statement that his company has already identified some new acquisition possibilities.

"As we have said all along, Mylan viewed Perrigo as a unique and exciting opportunity, but not one that was required for the future success of our company," he said.