Source: Flickr via Matt McGee

The National Football League is by far and away the most popular sport in the U.S. and has been so for 30-straight years, according to surveys taken by Harris Poll. The American public's love affair with the NFL has also made the league an extremely successful business, raking in a reported $9.2 billion in revenue in 2013.

With a growth rate that has averaged about 3.5% over the last decade, the NFL is on track to generate somewhere around $9.57 billion in revenue for 2014. Despite these staggering numbers, though, the NFL is no match for this single legal drug in terms of either revenue or overall growth.

Source: AbbVie

How Humira dwarfs the entire NFL
In 2013, AbbVie's (NYSE:ABBV) flagship anti-inflammatory drug Humira posted a little over $11 billion in sales, making it the world's best-selling drug. Perhaps most impressively, Humira's growth rate has exceeded double-digits every single quarter since AbbVie was spun-off from Abbott Laboratories in Jan. 2013. And, its incredible growth rate is showing no signs of slowing down. 

After gaining a recent approval for use in treating pediatric Crohn's disease patients 6 years of age and older earlier last year, Humira is now approved by the Food and Drug Administration for a whopping eight indications. The net result has been a growth rate that has climbed to a mind-boggling 17.8% (on an operational basis), year-over-year, in the third-quarter of 2013.  

Humira's total annual sales in 2014 should thus come in close to $13 billion, beating the NFL's projected yearly haul by an impressive 35%!

Humira is the key to AbbVie's jaw-dropping growth story 
Going forward, AbbVie's management expects Humira to post double-digit sales growth again this year, helping the company to grow revenues by an estimated 30% in 2015. 

This amazing sales growth has allowed management to reward shareholders by raising the dividend a princely 22.5% in the two years since the company became independent. AbbVie thus offers investors a noteworthy yield of 3.12%, one that easily tops the sectorwide average of 2.42%.

Moreover, Humira's rising revenues have provided plenty of cash for share buybacks, with AbbVie expected to repurchase over $5 billion worth of stock in the next few years.

Because of Humira's world-leading sales and staggering growth profile, AbbVie's shares have trounced the broader markets as well, shown below.

ABBV Chart

Humira's rocket-like sales growth may be coming to an end
Despite Humira's importance to AbbVie's top and bottom-lines, the drug is facing a serious threat. Specifically, the drug is scheduled to lose patent protection in the U.S. in Dec. 2016 and in the EU in 2018. 

Because Humira accounts for over 60% of the company's total sales at present, this looming patent expiration issue is certainly cause for concern. 

That said, AbbVie's management hasn't sweated it, pushing the drug into several additional clinical trials to continue expanding its label.

While it may seem like a bad idea to continuing investing heavily in a drug nearing the end of its market exclusivity, AbbVie is probably right for doing so. What's key to understand is that Humira is a biological-based medicine, and biologics are protected by extremely high standards in regards to the approval of generic rivals called "biosimilars" in the U.S.

In fact, the first biosimilar ever in the U.S. -- Novartis' EP2006 indicated as a biosimilar for Amgen's (NASDAQ:AMGN) Neupogen -- is only now closing in a regulatory approval, nearly 5 years after the regulatory pathway for biosimilars was put into place by the Affordable Care Act in 2010.

Although multiple companies are hoping to launch a biosimilar for Humira in 2017, the fact of the matter is that the nature of the regulatory pathway for biosimilars probably prohibits such an event from immediately taking place, giving the drug a few more years of exclusivity beyond its patent expiration.  

Foolish takeaways
The NFL is an enormously successful business by any measure. But top-selling medicines like Humira show just how large the pharmaceutical industry has become, and the drug's double-digit growth rate suggests that this trend is likely to continue for the foreseeable future. As a result, the outlook for innovative drugmakers like AbbVie is as bright as ever, making them excellent stocks to own for the long-haul.