Pfizer Inc's looming deal to buy Botox maker Allergan Plc for more than $150 billion will cap a record-breaking run for mergers and acquisitions in the healthcare sector, taking the cumulative value of deals in 2015 to more than $600 billion.

Helped by cheap finance, healthcare has seen an unprecedented wave of M&A activity since the start of 2014, stretching from large drugmakers buying up smaller rivals to consolidation among makers of generic medicines and tie-ups between insurers.

Pfizer's tax-driven takeover of Allergan, which people familiar with the matter said won board approval on Sunday, will vault healthcare into the top slot for deal-making by sector, ahead of both energy and technology.

Data shows healthcare M&A at the end of last week had already reached $460.2 billion, eclipsing the full-year record of $392.4 billion set in 2014. Energy and power M&A stood at $572.4 billion and high technology on $514.4 billion.

The Pfizer-Allergan deal is the largest ever in the healthcare sector, beating the previous record set in 1999 when Pfizer agreed to buy Warner-Lambert for $90 billion to gain control of cholesterol fighter Lipitor.

It will inevitably give executives at other large U.S.-based drugmakers pause for thought, since Pfizer will gain a relative advantage in tax terms. But industry officials and bankers question whether others will race to follow suit.

Merck & Co Inc, the second biggest U.S. drugmaker behind Pfizer, declared a year ago that similar so-called "inversion" deals, in which a company redomiciles in a country with lower taxes, would not fit its corporate strategy.

Swiss and British drug giants, meanwhile, already enjoy more favorable tax regimes and their executives have said recently they do not see mega-mergers as the way forward, even though many were created by such tie-ups in the 1990s and early 2000s.

GlaxoSmithKline Plc's strategy head David Redfern said last week that sheer scale was not needed for success, following suggestions that the British drugmaker might have made an alternative target for Pfizer, which failed to buy AstraZeneca Plc in 2014.

Redfern was speaking at a Financial Times pharmaceuticals conference where the majority of the audience voted in a poll that bolt-on deals were likely to dominate M&A over the next year.

Such mid-sized deals have come thick and fast recently, often to help larger firms replenish their product development pipelines. AbbVie Inc struck the biggest such deal in March when it agreed to buy Pharmacyclics for $21 billion to expand its reach in cancer.

But drug companies have also been divesting some assets, with Novartis AG and GSK finalizing a $20 billion asset swap in March, while Allergan is selling its generics drugs business to Israel's Teva Pharmaceutical Industries Ltd for $40.5 billion.