5. Merck & Co.

Merck has been growing in part thanks to the success of its megabluckbuster cancer drug Keytruda. (Merck)

2019 sales:
$46.84 billion
2018 sales $42.29 billion
Headquarters: Kenilworth, New Jersey

Propelled by immuno-oncology superstar Keytruda, Merck posted strong growth in 2019 and reached $46.84 billion in global sales. Merck was a standout performer among its Big Pharma peers, and now the company is shaking up its structure with a sizable spinoff.

Along with fourth-quarter results, Merck unveiled a spinoff of its women's health, legacy brands and biosimilars into a new company that's set to generate around $6.5 billion in 2020. The company expects the deal to be complete in the first half of 2021.

Like other pharma giants such as Pfizer and GlaxoSmithKline, Merck is offloading certain products to focus on its innovative medicines. In February, Merck CEO Ken Frazier said in a statement the company's 2019 performance—and its 2020 guidance—show Merck is in a "position of operational and financial strength."

“It is this position of strength, born of our focused execution, that gives us the confidence to spin off" the new company, he added.

Within Merck's pharma group last year, Keytruda continued to soar as it scored new approvals and scooped up market share. Last year, its sales reached $11.1 billion, a 55% jump from 2018. Keytruda is by far the company's best-selling product, followed by aging Type 2 diabetes medicine Januvia, which generated $5.52 billion last year.

RELATED: Merck preaches the value of 'focus' as it spins off $6.5B women's health, biosimilars units

5. Merck & Co.