BioPharma Dive strives to provide insight into the inner workings of the industry, giving context to the headlines. In that spirit, we have compiled data on biopharma drug sales which we think will help support and frame our daily coverage. 

These charts are updated each quarter with the release of new public data on revenues and spending. A companion article, covering R&D spending, can be viewed here. 

Best-selling drug sales

Except for the largest pharma companies, most biotechs and mid-sized drugmakers rely on one to two blockbuster therapies to fuel the majority of their revenues. Each top-earner is at once a success story — earning billions in annual sales — and a future vulnerability, as overdependence on one drug for too long can lead to a painful adjustment down the road. 

Across the industry, a handful of major companies are facing challenging transitions away from aging flagship products to newer, less established medicines. 

Four of the following charts show bestseller sales as a percent of total sales for 16 leading biopharmas. Others highlight the performance of specific drugs. 

The writing has been on the wall for Gilead, as the pool of easily reached, untreated hepatitis C patients has shrunk considerably since the introduction of the biotech's highly effective antiretrovirals Harvoni (ledipasvir/sofosbuvir) and Sovaldi (sofosbuivr). But the decline in sales of Harvoni has been more rapid than many anticipated. 

In January, Gilead dramatically slashed its sales forecast for 2017, predicting combined sales of Harvoni and Sovaldi would reach between $7.5 billion to $9 billion for 2017, significantly less than the $14.8 billion Gilead earned from the two in 2016.

Amgen's Enbrel (etanercept) has been one of the best-selling drugs in the world, along with other TNF alpha inhibitors like AbbVie's Humira (adalimumab) and J&J's Remicade (infliximab). 

But sales fell by 15% in the first quarter, compared to the same period a year prior. Amgen was quick to wave off concerns, citing weakness in the rheumatology and dermatology sectors during the first three months of the year. Yet, some wonder whether it may be a sign of further wobbles to come, as competition from biosimilars and new drugs like Novartis' Cosentyx (secukinumab) heats up. 

While big pharmas still face threats from generic and biosimilars competition, their large diversified portfolios shield them from a sharper impact to bottom lines. 

Sales of Novartis' Gleevec, for example, have fallen dramatically over the past year and half as generics have come on line. But that blow has largely been absorbed by growth from the Swiss pharma's newer drugs. 

The future of both Bristol-Myers Squibb and Merck hinges on immuno-oncology. Both Bristol-Myers' Opdivo (nivolumab) and Merck's Keytruda (pembrolizumab) have quickly become blockbusters. As competitors have flooded in, however, the jockeying for positioning has become more consequential. 

Bristol-Myers misstep last summer in lung cancer looks to have ceded the advantage to Merck and Keyturda, which has narrowed Opdivo's commercial lead. But if anything is certain in immuno-oncology, it is that fortunes can change fast. 

Declining sales for now-generic Crestor (rosuvastatin) and Nexium (esomeprazole), while expected, have upped the pressure on AstraZeneca — which hopes to make good on CEO Pascal Soriot's promise of $40 billion plus in revenues by 2023. 

Respiratory mainstay Symbicort (budesonide/formoterol) is now the company's top-selling drug, but that too has faced pricing pressure in the U.S. 

AstraZeneca hopes it can recharge growth through oncology drugs like Imfinzi (durvalumab) and a new wave of biologics drugs in respiratory diseases.