4. Advair/Seretide
The company: GlaxoSmithKline
Worldwide sales: $8.25 billion

You'd be hard pressed to find another drug that has kept the patent cliff at bay quite like Advair has. Its difficult-to-copy Diskus inhaler technology has foiled some drugmakers in their attempts at a copy and scared others off from even having a go at it. And so while Advair lacks IP protection in several countries, sales of the drug--known as Seretide outside the U.S.--swelled 4% on the year.

An 8% U.S. jump buoyed that growth, with sales there tallying £2.8 billion ($4.6 billion) in 2013. But Advair's U.S. dominance won't last forever, and reports this year have showed signs of slipping for the behemoth. According to Bloomberg, the drug's market share went from 67.2% for the last full week in October to 61.6% for the week ended Jan. 24; longtime rival Symbicort from AstraZeneca ($AZN) now boasts a 30% piece of the pie, while Merck's ($MRK) Dulera has grabbed up 8.4%.

But as far as U.S. copycats go, they're a long way off--if you ask generics giant Teva Pharmaceutical Industries ($TEVA). Last October, the Israeli company declared that it doesn't expect to see true substitutes for the GlaxoSmithKline ($GSK) best-seller before 2018. That assuaged some investor worries brought on by new FDA guidance for the development of generic combo inhaled drugs like Advair, which required only "relatively basic" preclinical tests and a short clinical trial.

In Europe, though, generic competition is more immediate. Though knockoffs there aren't substitutable--meaning they have to compete with Advair on the market as branded rivals might--they're gaining ground. In particular, a new med from Novartis' ($NVS) Sandoz unit, dubbed AirFluSal Forspiro, is on an approvals tear, now boasting regulatory green lights in a handful of EU countries, including Germany. And even without AirFluSal's impact--it received its EU nod in mid-December--sales of Seretide declined in 2013, dropping 2% to £1.5 billion ($2.4 billion).

For more:
Special Report: The top 10 pharma companies by 2013 revenue - GlaxoSmithKline
GlaxoSmithKline's Advair loses share to AstraZeneca's aggressive Symbicort discounts
As GSK hails its FDA nod for Anoro, a Novartis Advair knockoff prepares for its close-up
FDA helps generic drugmakers get copies of Glaxo's Advair to market
Teva: Copying GlaxoSmithKline's Advair too tough for success before 2018
New FDA guidance threatens GlaxoSmithKline's Advair

-- Carly Helfand (email | Twitter)