6. GlaxoSmithKline

GSK posted a slight sales increase despite continued pressure on its respiratory blockbuster Advair. (Eric Sagonowsky)

2018 revenue: $40.99 billion (£30.82 billion) 
2017 revenue: $39.24 billion (£30.19 billion) 
Headquarters: Brentford, U.K. 

GlaxoSmithKline is in the midst of a transformation after working to exit oncology a few years ago. As it ramps back up in the field, the company is facing competitive pressure on old drugs and working to make the most out of new launches.

Overall, GSK pulled in £30.8 billion last year ($40.96 billion), up 5% at constant exchange rates from 2017’s £30.2 billion. In its key respiratory business, Glaxo is no stranger to competition. The company has been discounting Advair for years to retain market share, resulting in a multibillion-pound loss over the period. But in February, Mylan launched an Advair generic that’ll add to the pain this year.

Advair isn't alone in GSK's respiratory portfolio, though. 3-in-1 drug Trelegy pulled in £156 million last year, its first full year on the market, and asthma biologic Nucala leapt 66% at constant exchange rates to £563 million. The thing is, Nucala's facing a formidable rival of its own in AstraZeneca's Fasenra, a drug GSK executives have acknowledged they underestimated. And late last year, Sanofi got in on the action with a competing asthma nod for Dupixent, a drug some analysts have said could generate $2.5 billion in that indication alone.

Aside from respiratory, GSK is also working to flex its muscle in HIV. Overall, the company’s HIV unit turned in 11% growth at constant exchange rates to £4.7 billion. Dolutegravir-based regimens—Triumeq, Tivicay and Juluca—contributed £4.4 billion, up 16% at constant exchange rates versus the prior year. But again, the British drugmaker is up against big competition going forward—this time from Gilead, whose Biktarvy has been tearing up the market. 

RELATED: As Advair generic hits, GSK looks to oncology to spur growth alongside Shingrix, HIV 

Market watchers won't forget another big launch at GSK last year, though. Shingles vaccine Shingrix, approved in October 2017, had a massive introductory year in the U.S., passing the blockbuster threshold despite supply constraints. The company has pledged to boost capacity, as demand has been higher than anticipated.

And if all goes according to plan, it won't be long before cancer drugs are contributing significantly to Glaxo's top line. A few years ago, GlaxoSmithKline wanted out of oncology, and it made a massive asset swap with Novartis to accomplish that goal. Fast-forward to today: Under CEO Emma Walmsley, the London drugmaker has done a 180-degree turn and is building out its pipeline in the hot treatment field. In recent months, it’s inked a couple of multibillion-dollar pacts to show it’s serious about competing in oncology. 

RELATED: With Tesaro in the bag, GSK's cancer pipeline is pumping again, CEO says

Late in 2018, GSK made a splash with its Tesaro buyout, getting its hands on PARP inhibitor Zejula plus pipeline drugs in a $5.1 billion deal. Then, in February, GSK inked an immuno-oncology R&D deal with Merck KGaA that could be worth €3.7 billion ($4.2 billion).

Looking ahead, the company expects a slight sales decline for its pharma business this year due to new Advair generics. At the same time, the company will likely see a sales boost in its consumer business after purchasing Novartis' share of a joint venture. Last March, GSK said it'd offer up $13 billion to buy Novartis' 36.5% stake in the JV. Then, late in 2018, GSK and Pfizer said they'd join together to form a world-leading consumer unit. A few years down the line, GSK plans to split out the consumer business from its pharma and vaccines entities.

GSK executives told analysts the company expects a mid- to high-single-digit decline in its earnings per share in 2019.

6. GlaxoSmithKline